Western Cue Bid
Ideally, you should have a half-stopper (Qxx or Jxxx) in the suit yourself, though if you think your side can take 9 tricks off the top, you could make the cue bid with as little as xx in the suit.
Example: 1-(1)-2 with a hand like
Responder can bid 3N with as little as Kx in the suit since he can win the opening lead in the suit with his King, then your remaining Qx stops your RHO's AJ...
This should not be confused with cue bidding a suit in an obvious attempt to get partner to bid NT if he has a stopper in a higher unbid suit. For example, playing 2-Over-1 Forcing, the bidding might go 1-2, 3-3 where 3 asks partner to bid NT if he has a stopper in Spades.