It's an area within your home, typically with a large counter or bar, where you can prepare drinks and store your various types of liquors, drinks, and mixers. Essentially, a wet-bar has a sink with running water. A dry bar only has a place where you can prepare drinks and no sink.
A wet bar is a space in a home or hotel room that provides a countertop area to prepare alcoholic or non-alcoholic drinks. The addition of small appliances such as microwaves or mini-fridges, or even large built-in ones such as ovens are additions you may consider adding to a wet bar.