Once you lock, rate movements don't change your rate — as long as you close before the lock expires. Lock periods are typically 15, 30, 45, or 60 days. Longer locks usually carry a small premium in rate.
If your close gets delayed past the lock expiration, you can usually extend (for a fee) or re-lock at current market (risky if rates have moved against you).
"Float down" options exist at some lenders — if the market improves meaningfully during the lock period, you can relock at the lower rate for a fee. Ask about float-down on any lock longer than 30 days.