Cue bid

In contract bridge, a cue bid (also, cuebid or cue-bid) is a term that applies to two types of bid:

After the opponents have bid a suit, a cue bid of that suit is normally intended as a forcing bid. It shows interest in contesting the contract and asks partner to describe his hand.

An immediate cue bid is made directly over opponent's opening bid. Traditionally, it denotes a hand unsuited for a takeout double. For example, after RHO opens 1♠, a hand such as ♠ — ♥ AKQ10985 ♦ AQ6 ♣ K85 would prefer not to double for takeout, because partner might make a penalty pass. A cue bid of 2♠, as traditionally used, would be appropriate: it tends to show great high card strength, probably with a hand pattern unsuited to defense. Partner is expected to respond in his longest suit, and the subsequent bidding proceeds naturally.

However, those very strong hands are rare enough that the traditional meaning has been largely abandoned, and other meanings assigned to the immediate cue bid. The most common treatment is now the Michaels cuebid, which shows a weakish or moderate hand with at least 5-5 in two unbid suits.

The immediate jump cue bid of opener's suit has a specific meaning. It is typically a long totally solid minor with stoppers in the other two suits. Partner is asked to bid 3NT with a stop in the suit opened or else to bid four or five clubs (pass or correct).

Generally, after the opponents have bid a suit, a cue bid of that suit shows strength, and forces the bidding to continue for at least one round. The following are common situations:

Once a trump suit has been agreed and the bidding cannot die below the game level (e.g. 1♥–3♥, or ... 2♥–3♥, or 1♥–1♠; 3♠), any subsequent bid of a suit other than the trump suit is a cue bid showing first round control of that suit, i.e. the ace or a void.

...
Wikipedia