Once you've begun the mortgage process, your loan officer will offer you the opportunity to lock in your interest rate. This freezes your quoted mortgage rate for a set period of time, during which you’re expected to get all your documents in order and close.
Miss that window and your rate will be affected by current rate fluctuations and could potentially rise. Right now, rates are still on the low side, historically speaking. But they've been creeping up over the course of the last year, and are expected to continue the trend.