A convention played after an opener’s reverse following a one-over-one response. The lower of the fourth suit and 2NT is an artificial weak bid, saying responder is happy to stop short of game if opener has a minimum reverse; other responder’s rebids create a game force.
Using Blackout, 2NT here is the weak action. Opener usually bids 3♣ over 2NT, allowing responder to pass (if wishing to play in 3♣) or sign off in another suit.