A fixed rate is an interest rate that will not change for the duration of the period you fix for e.g. 3 year fixed rate at 3.5% the interest rate will be 3.5% for the next 3 years when you can then choose a fixed rate again (at the rate on offer at the time that may be higher or lower) or chose a variable rate.
Can I switch rates (i.e. remortgage) if I am part-way through a fixed rate period?
If you are part-way through a fixed rate period it is possible to remortgage to a new rate however you may be subject to a break penalties / costs. If you contact your existing mortgage provider they will confirm if a break penalty would apply and the amount of any such penalty.