A default action is an action a player is allowed to take, should it reach their turn and they decide not to want to give a clue. While it is not necessary for systems to have default actions to perform well, most players find it simpler to have them.
Most popular systems use a default action that is discarding a card, and they call this card the chop.
The alternative to systems prescribing default actions is a default lock assumption: if a player is told nothing about their hand then they should give a clue: no action the player could take is guaranteed to be safe.