as the name says, your fixed-rate mortgage locks in the rate you have at closing. compared to a variable rate mortgage, which has a rate that is adjusted to the market, you have the certainty that at least your rates won’t go up.
that being said, it is possible for other parts of your payment to change, including taxes, insurance, etc.

yet another moving part in addition to rates and fees associated with the loan and closing is the point system. most lenders allow borrowers to buy ‘points’, which is essentially money up front for a lower rate.
points are one-time fee
the cost for one point is equal to 1% of the total loan amount.
that one point will drop your rate by 0.25%
should you buy points? one simple way of looking at it is knowing when you’ll pass the breakeven point. typically 5-10 years in or so, the amount you saved on lowering your monthly payment will “break even” with the amount you put upfront.
for some then, buying points is one way to lower monthly payment and save you over the life of the loan.

in a nutshell, apr is the cost of your total loan credit, calculated into an annual interest rate. apr can be a comparison tool to help your get closer to an ‘apples to apples’ comparison between different loans with various fees and rates.
for example, you could have 2 loans for the same $500,000 amount.
loan 1 has a rate of 4.25%, with $6,000 in fees.
loan 2 has the same $6,000 in fees, but includes 1 point purchased for 1% of the loan amount, or $5,000. that brings the rate down to 4% even, with $11,000 in closing costs.
loan 1 has an apr of 4.35%, based on the added fees. loan 2 has an apr of 4.18%. that means factoring in the principal, interest, and upfront fees, loan 2 will have a lower cost over the life of the loan.