Guard crush (or occasionally “guard crash”, thanks in no small part to both English words being pronounced the same way in Japanese) is the act of forcefully undoing an opponent’s block, generally through the accumulation of guard crush values. Hence, blockstrings intended to attain this feat are also called “guard crush”.
When someone gets guard-crushed, they will be left defenseless for a certain (generally short) amount of time. It is still up to the aggressor to exploit this.
The first game with guard crush was World Heroes Perfect, which had specific attacks that could do this.
Guard crush is a highly apparent system in the KOF series, with the obvious intention of discouraging excessive blocking (i.e., KOF emphasizes rushdown over turtling). Within the series, some moves will perform an instant guard crush if blocked.