Cue-bids are a way of exchanging information en route to a possible slam.
We already know of one method of slam bidding (Blackwood) which is question-and-answer session regarding the number of Aces and Kings partner holds. Sometimes, though, Blackwood does not help us locate whether we might be missing two quick losers or not. The essential question might be "Does partner hold the right Ace?"
A cue-bid shows a control in a side-suit and invites partner to co-operate by showing where his controls are. Rather than tying partner down and asking questions (which is what Blackwood does) a cue-bidding sequence is essentially a co-operative venture; a conversation between partners.