Mortgage interest rates are a mystery to many of us—whether you’re a home buyer in need of a home loan for your first house or your fifth.

After all, what does “interest rate” even mean? Why do rates swing up and down? And, most important, how do you nab the best interest rate—the one that’s going to save you the most money over the life of your mortgage?

Here, we outline what you need to know about interest rates before applying for a mortgage.

Why does my interest rate matter?

Mortgage lenders don’t just loan you money because they’re good guys—they’re there to make a profit. “Interest” is the extra fee you pay your lender for loaning you the cash you need to buy a home.

Your interest payment is calculated as a percentage of your total loan amount. For example, let’s say you get a 30-year, $200,000 loan with a 4% interest rate. Over 30 years, you would end up paying back not only that $200,000, but an extra $143,739 in interest. Month to month, your mortgage payments would amount to about $955. However, your mortgage payments will end up higher or lower depending on the interest rate you get.

Why do interest rates fluctuate?

Mortgage rates can change daily depending on how the U.S. economy is performing, says Jack Guttentag, author of “The Mortgage Encyclopedia.”

Consumer confidence, reports on employment, fluctuations in home sales (i.e., the law of supply and demand), and other economic factors all influence interest rates.

“During a period of slack economic activity, [the Federal Reserve] will provide more funding and interest rates will go down,” Guttentag explains. Conversely, “when the economy heats up and there’s a fear of inflation, [the Fed] will restrict funding and interest rates will go up.”

How do I lock in my interest rate?

A “rate lock” is a commitment by a lender to give you a home loan at a specific interest rate, provided you close on your home in a certain period of time—typically 30 days from when you’re pre-approved for your loan.

A rate lock offers protection against fluctuating interest rates—useful considering that even a quarter of a percentage point can take a huge bite out of your housing budget over time. A rate lock offers borrowers peace of mind: No matter how wildly interest rates fluctuate, once you’re “locked in” you know what monthly mortgage payments you’ll need to make on your home, enabling you to plan your long-term finances.

Naturally, many home buyers obsess over the best time to lock in a mortgage rate, worried that they’ll pull the trigger right before rates sink even lower.

Unfortunately, no lender has a crystal ball that shows where mortgage rates are going. It’s impossible to predict exactly where the economy will move in the future. So, don’t get too caught up with minor ups and downs. A bigger question to consider when locking in your interest rate is where you are in the process of finding a home.

Most mortgage experts suggest locking in a rate once you’re “under contract” on a home—meaning you’ve made an offer that’s been accepted. Most lenders will offer a 30-day rate lock at no charge to you—and many will extend rate locks to 45 days as a courtesy to keep your business.

Some lenders offer rate locks with a “float-down option,” which allows you to get a lower interest rate if rates go down. However, the terms, conditions, and costs of this option vary from lender to lender.

How do I get the best interest rate?

Mortgage rates vary depending on a borrower’s personal finances. Specifically, these six key factors will affect the rate you qualify for:

1. Credit score: When you apply for a mortgage to buy a home, lenders want some reassurance you’ll repay them later! One way they assess this is by scrutinizing your credit score—the numerical representation of your track record of paying off your debts, from credit cards to college loans. Lenders use your credit score to predict how reliable you’ll be in paying your home loan, says Bill Hardekopf, a credit expert at LowCards.com. A perfect credit score is 850, a good score is from 700 to 759, and a fair score is from 650 to 699. Generally, borrowers with higher credit scores receive lower interest rates than borrowers with lower credit scores.
2. Loan amount and down payment: If you’re willing and able to make a large down payment on a home, lenders assume less risk and will offer you a better rate. If you don’t have enough money to put down 20% on your mortgage, you’ll probably have to pay private mortgage insurance, or PMI, an extra monthly fee meant to mitigate the risk to the lender that you might default on your loan. PMI ranges from about 0.3% to 1.15% of your home loan.
3. Home location: The strength of your local housing market can drive interest rates up, or down.
4. Loan type: Your rate will depend on what type of loan you choose. The most common type is a conventional mortgage, aimed at borrowers who have well-established credit, solid assets, and steady income. If your finances aren’t in great shape, you may be able to qualify for a Federal Housing Administration loan, a government-backed loan that requires a low down payment of 3.5%. There are also U.S. Department of Veterans Affairs loans, available to active or retired military personnel, and U.S. Department of Agriculture Rural Development loans, available to Americans with low to moderate incomes who want to buy a home in a rural area.
5. Loan term: Typically, shorter-term loans have lower interest rates—and lower overall costs—but they also have larger monthly payments.
6. Type of interest rate: Rates depend on whether you get a fixed-rate mortgage or an adjustable-rate mortgage, or ARM. “Fixed-rate” means the interest rate you pay remains fixed at the same level throughout the life of your loan. An ARM is a loan that starts out at a fixed, predetermined interest rate, but the rate adjusts after a specified initial period (usually three, five, seven, or 10 years) based on market indexes.

Tap into the right resources

Whether you’re looking to buy a home or a homeowner looking to refinance, there are many mortgage tools online to help, including the following:

• A mortgage rate trends tracker lets you follow interest rate changes in your local market.
• A mortgage payment calculator shows an estimate of your mortgage payment based on current mortgage rates and local real estate taxes.
• Realtor.com’s mortgage center, which will help you find a lender who can offer competitive interests rates and help you get pre-approved for a mortgage.

For more smart financial news and advice, head over to MarketWatch.