Clearance is about access. A pawn or piece may occupy the very square from which a queen would give mate, or block the diagonal a bishop needs to join an attack. By sacrificing the obstructing unit — often with tempo, via a check or capture — the attacker clears the path for the follow-up blow.
These ideas are easy to miss because the move looks like it throws away material for nothing. The key is to visualize the position after the obstruction is gone: if a different piece then delivers a winning threat along the cleared line, the sacrifice is justified. Clearance combinations are a staple of brilliancy prizes.