What does Rainbow mean in poker?

Rainbow in poker means the board cards are all different suits, so there is no flush draw yet.

How does it work?

The term is used most often for the flop. If the flop comes with three cards of three different suits, it is called a rainbow flop. That board texture usually reduces the threat of a made flush draw and changes the way players judge pressure, continuation bets, and range strength. The term is simple, but important, because the suit texture of the board directly affects how the hand is played.