An American idiom:
1. to appeal to the senses so much that one wants to consume what is seen and/or smelled. The apple pie in the oven makes SonicQ's mouth water. (The apple pie smelled so good that SonicQ wanted a piece- or more.)
2. to be quite attractive so that one desires to have something or someone. Webster 1913 makes her mouth water. (Webster 1913 makes her want him.)