An underfull refers to a non-nut full house or in some cases simply to a full house with a three of kind lower than the pair.

In a game of hold'em the board may be showing 8♣ 8♦ 5♦ 5♠ A♦. A player holding a five would have the underfull as any player with an eight would have a superior full house. Though even that player could be in danger of a player with pocket aces which forms the nut full house, the overfull.