Lock in interest rate?

The loan officer wants us to lock in the interest rate @ 4.875% today. Is this good enough offer? Also I have a question about locked in interest rate. If the interest rate drop down more let's say to 4.25%, can I change to the new rate? Somehow I have the impression that the locked in is lock up for the up limit but can go down freely. If that's the case, I should lock it in now. Thanks