Pawn break
A pawn break is a pawn advance that challenges the opponent’s pawn chain and opens files or diagonals.
Closed positions stay closed until someone hits a pawn with a pawn. The break is the move that starts the trade. After the dust settles, pieces that were locked in get files and diagonals.
c4 is the Queen’s Gambit break against …d5. White offers a wing pawn to pull Black’s center pawn aside, or to keep a two-pawn center if Black refuses.
In the Sicilian the break is …d5; in the French it is …c5; in the King’s Indian it is …f5 or …c5. The coach names a pawn break when the advance actually challenges a chain, not when you just push a random pawn.