A dry board refers to community cards that are disconnected from each other and do not give opportunities for players to make straight or flush draws.
This term is used in community card poker games like Texas Holdem and has the opposite meaning of a wet board.
For instance, Kc8s2h is a dry board, which is also a rainbow since there are no connected hands you can have here. If the turn card is 5d, that is also a brick that keeps the board dry. On the other hand, Ts9s7c would be considered a wet board since it has more potential draws, so players are likely to have more hands worth continuing with.
