Adults-only resorts do not allow guests under a certain age. This could be for the entire property, or some hotels simply have an adults-only wing. The minimum age varies -- usually 16, 18 or 21.
It basically means there are no children around and everything caters to grownups enjoying an age-appropriate atmosphere conducive for peace, romance or partying. These types of resorts are popular with couples enjoying a romantic getaway, singles and groups of friends on a fun trip, bachelor and bachelorette parties.