A fixed rate period, typically used in mortgages is a set period of time where the interest rate is fixed, it does not change for the whole period of time.
Typically in the UK, a mortgage would have a 2, 3 or 5 year period at the start of the mortgage where the interest rate is fixed (normally at a low rate), this is the fixed rate period.
After this period ends, the mortgage rate reverts (changes) to the lenders standard variable rate (SVR), which is typically higher.
If you have a fixed rate mortgage, you should be looking to remortgage at the end of your fixed rate period to avoid moving to the standard variable rate, which is typically much higher.
Learn more about mortgages.