1. If you have small cards to guard an honour in a suit, you may describe the honour as ‘protected’.
2. To bid after two successive passes so that your partner may have another opportunity to bid. The word protect comes about because your partner may have held adequate values for a bid but had the wrong kind of hand (such as length in the opposing suit) to act on his own.
3. If you prevent an opponent from leading through a vulnerable high card, you protect that high card.
South’s king is safe from a lead by West, but not by East. To protect the king South might choose a bid that makes him rather than North declarer.
During the play, South might arrange to lose the lead to West rather than East.