The wet board term is a reference to the “wet” texture of the community cards. In other words, this is to say that they’re not firmly set in a few possible combinations.
Instead, wet boards provide many different potential winning combinations. This is the opposite of a dry board.
For example, a board of 7, 8, and 9 in which two of these cards are of the same suit is considered a wet board.
This is because it provides players with many different possibilities of making either a flush or a straight.