The wide part of a knife’s handle, at the place where the blade joins it, which prevents one’s hand from slipping onto the blade if the knife meets resistance while being pushed forward.  Some folding knives have large T-shaped bolsters as guards; others have a “folding guard” pivoted on the blade such that it folds flat against the handle when the blade is closed.

Bowtie pattern knife with fixed guard

Case 61050L folding hunter with lockback feature and folding guard.