Wet board refers to connected community cards with multiple draw opportunities for players to hit a flush or a straight.
For example, the flop of 8s 7s 6c is considered a wet board since players can already have made hands like straight or two pairs. On top of that, there are multiple opportunities to hit straights or flushes with the next community cards on the turn and river, creating entirely different dynamics compared to dry boards.
If you are a preflop aggressor, you should proceed carefully in these situations and bet less frequently but use bigger sizing when you do so since wet boards usually favor the caller. It is also worth mentioning that one pair hands lose a lot of value in these situations since there are multiple ways to outdraw such holdings or force them to fold by putting a lot of aggression.