A buckle is a clasp or fastener, such as the one on the end of a belt. In the old days, not only belts but also shoes had buckles.
When you fasten your belt buckle, you can say you buckle it — you also buckle your seat belt and buckle a kid's bike helmet under his chin. When the ground buckles during an earthquake, it folds or bends out of shape, and when your knees buckle, you could fall. You might also buckle under pressure or stress. The Latin root word is buccula, "cheek strap of a helmet."