Closed and semi-closed positions can feel static until a pawn break changes everything. By offering to trade pawns, a break can open a file for your rooks, a diagonal for your bishop, or strike at the base of an enemy pawn chain. Typical examples are ...c5 and ...f6 against a French center, or ...d5 to challenge a big White center.
Timing is everything: the same break can be strong or weak depending on whether your pieces are ready to exploit the lines it opens. A central principle of strategy is to identify the correct pawn break for the structure and to prepare it fully before playing it.