A convention whereby an overall in no-trumps which, given the previous bidding, could not logically be natural, is used instead to show length in the two lowest-ranking unbid suits.
The most popular application is as an immediate overcall of a suit opening, although this is an extension of the convention, for such a bid could logically be natural.
Look at the two sequences below:
In the first sequence, West cannot possibly hold a very strong balanced hand.
In the second sequence, it is also usual to play that the 2NT bid shows the minors, because, while West could hold a very strong balanced hand, a two-suited hand is much more likely. West might hold: