Source: http://www.bridgeguys.com/
This highly useful concept can best be explained with an example. Say you open the bidding with one heart, holding:
Opener: 
x x 
A K x x x 
x 
K J 10 x x
Your partner responds one notrump and your right hand opponent bids two spades. Despite your mere 11 high card points you’d love to bid three clubs. After all, you know the opponents have at least eight spades (partner denied four when he bid one notrump), so you can’t leave them on the two level. The problem with bidding three clubs is that you would also have to bid three clubs with a 17-count! How does partner know if you’re just competing, or if you really have a good hand? This kind of problem comes up all the time – you’d love to bid, but you’re afraid to mislead partner about the strength of your hand. We solve this problem with Good-Bad 2NT – a variation of the Lebensohl convention. It enables us to compete to the three level in a very effective manner. Here is the rule:
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In a competitive auction, when RHO makes any two level call, our 2NT bid is not natural.
It shows the desire to compète to the three level and requests partner bid three clubs
after which the 2NT bidder shows his suit.
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If the suit is clubs, the two-notrump bidder can pass the forced three-club bid. Bidding directly on the three level (without relaying first with two notrump) shows extras.
Here are examples illustrating the usefulness of Good-Bad 2NT:
West North East South 1 2 2 ?
Kxx Axx Q109x xxx
Here, you can bid three diamonds and partner will know that you have a good three-diamond raise. If you held only:
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