Signals made by a defender to encourage a continuation of the suit led, or to suggest a switch. By tradition, you do this by playing a high card to encourage and a low card to discourage.
You are East and South is in 3NT. If the lead is the ♥K, you play the ♥7 to say you like hearts. If, however, the lead were a spade honour, you would follow with the ♠2 to say you have no help in that suit.