This is another way to try and improve bidding after a 5-card major opening, but unlike the artificial 2C response this convention is very simple. It involves opener's 2NT rebid after a 2-level response, for example 1H : 2D , 2NT. Particularly if you play a strong NT opening and often open 1NT on balanced hands with a 5-card major, the natural meaning for 2NT does not get used very much. Minimum balanced hands should be happy rebidding the major instead.
A more useful meaning for the 2NT rebid is showing a hand with 6+ cards in the suit opened and better-than-minimum values. These hands are extremely difficult to bid if you do not have such a convention available. In Acol, you would have to jump to the 3-level. This is problematic because it takes up so much space: there is a well-known problem with trying to decide which of responder's rebids should be cue-bids and which should be natural, and the reason this is so difficult is because there is not enough space to have both things. In 2/1, the jump to the 3-level promises an excellent suit, and hands without such a good suit have to rebid the major at the 2-level. It then becomes virtually impossible to show the extra strength of the hand.
The conventional 2NT rebid solves all these problems. It describes the important features of the hand immediately, while leaving plenty of space for further exploration. The best thing is that the continuations are very simple: all of responder's rebids at the 3-level are natural. For example, after 1H : 2D , 2NT if responder wants to set hearts as trumps and start cue-bidding he simply bids 3H. Whereas, if he wants to show a two-suiter or rebid his diamonds, he can do those things too. A jump to 4C would be a splinter.
The main drawback to this convention, apart from having to find another bid with a natural no-trump hand, is the possibility of wrong-siding a no-trump contact. However, because the bid promises a 6-card major, there is a high probability that the hand will be played in the suit instead. I feel that the advantages of being able to show the hand more than make up for this occasional problem.
This really is one of my favourite conventions. It can be added to most 5-card major systems (though it is not so good with a weak NT) and requires hardly any partnership discussion apart from the basic definition.