LEBENSOHL

When partner opens 1NT and RHO bids a suit you may just want to play in a 6 card minor at the 3 level. The only problem is that if you bid 3 of a minor directly it is FORCING. So what's to do? The secret is to use 2NT as an intermediate bid (sometimes known as a "puppet"). This warns partner you have no interest in game. Partner MUST first bid 3C (like a transfer) which you can then either pass or CORRECT to your suit. Partner should then Pass.
This is the basis of the LEBENSOHL convention. It is a system for dealing with intervention after 1NT when you have no interest in Game. It follows then that you can use direct 3 level bids to force partner to bid on.
This is only part of the system, but you should be aware of it as it is very common in club bridge. It also has other uses which are far more complex and should only be used in a practised partnership!
There are also other situations where 2NT no longer has its normal meaning. This is because in competitive auctions, 2NT is rarely a useful bid, as the key is usually to locate a sensible suit fit, and if you have a 2NT (10 - 12 balanced) hand you should probably be doubling for penalties. For instance, after a weak 2 opening, there is a difference between
2S (weak) - X - Pass - 3H

and

2S (weak) - X - Pass - 2NT
Pass - 3C  -  Pass - 3H

The second sequence shows no interest in Game (0 - 8), while the first is a decent (ish) hand and suit. The reason is that the Weak 2 has deprived your side of bidding space. If the opening bids had been 1S - X - Pass, you have room to show a good hand (9+) by jumping to 3? and bidding 2? with anything else (0-8).
This kind of differentiation makes life much easier for partner to make further, correct, decisions later in the auction. And it is of course our prime duty to keep partner well informed and very happy. We cannot play Bridge alone, so we must communicate accurately if partner is to succeed. For partner's success is also our success.

Enjoy

Kit