This is a chess opening system for White which, rather than immediately attacking the center and trying to establish an advantage straight away, aims to quietly complete development and only afterwards force an opening of the center.
The idea of this move is to solidify the centre and provide a stable shelter behind which White completes his development. This pawn will be pushed again in a few moves to e4, when Black will be faced with the dilemma of whether to capture it, enabling White's pieces to emerge, or allow it to advance further, where it can form the basis for an attack on Black's king. Economy of movement is the key to the Colle System.
A typical game might proceed as follows:
6.Nd2 c5 (Black tries to stake a claim in the center)
This is an ideal position for White from the opening moves of the Colle System. Black has done nothing to counter White's modest and economical preparations for the e4 pawn push, and after the likely exchanges in the center, White should be left with a small but comfortable advantage which can be used either to pursue an attack on the kingside, or to make further gains in the center. After e4, the White pieces that previously seemed cramped spring to life in quite a harmonious way.
More challenging Black strategies against the Colle System include the fianchetto of the white-squared bishop, or an early attempt to push the e6 pawn to e5. The idea is to attack White's center before he has a chance to consolidate and open it to his own advantage.