Against no trumps, on the lead of declarer’s first suit, the defenders play high or low to indicate their interest in the suit led at trick one.
Some play that high asks for a continuation while others play that high asks for a switch and some pairs play that the opening leader plays high to ask for a switch while the leader’s partner plays high to ask for a continuation – it is a matter of partnership agreement.
South plays in 3NT. West leads the five of spades, covered by the jack, queen and king. When declarer crosses to the ace of clubs to take the diamond finesse, East follows with the eight of clubs to indicate a desire for a spade continuation. Without the ten of spades but perhaps with the ace of hearts, East would follow with the two of clubs, asking for a switch.
If you play Smith Peters, it is usual to play count signals at trick one and show whether you like the suit via a Smith signal later.