Seems pretty self explanatory.
Up to 25k in points. Find a hotel you wanna stay at for 1 night, check the points value, as long as it's < 25,001 you've got a free night. When you go to "pay" there'll be a space to either enter a code from this cert, or it'll just say "pay with cert" or something.
Really don't know how to explain it any better than that.