An escape mechanism after an opponent doubles your partner’s 1NT opening.
You redouble to show an unspecified 5-card suit; opener must bid 2♣. You then show your long suit, passing 2♣ if it is clubs. Immediate bids after 1NT doubled show two-suited hands (possibly with only 4-4 in the suits) with the suit bid and a higher-ranking suit.
After the two suited-bids, opener passes with some fit for the suit bid and otherwise bids the next suit up to find out responder’s second suit.