The move d4 is an example of a break : an opening of the position. A Pawn break is a move which leaves the moving Pawn under attack by an enemy Pawn : the Pawns attack each other.
Breaks must be prepared properly. If a break is played too early, it loses much of its force and may even lead to a disadvantage.
If we look at the previous diagram, White will continue to prepare the break d4 until it is supported by the maximum number of pieces. This often occurs around moves 10-12 of the game, which is the time it takes to develop the White pieces which support the break.
White always has another possibility. Instead of playing d4, White can play d3, avoiding the break. This leads to positions where both players maneuver to prepare Pawn breaks on other squares.
