The word buckle has two main meanings:

1. A clasp or fastener that holds two ends of a strap together. You use buckles every day: on your backpack straps, your belt, or your seatbelt. A buckle typically has a frame with a prong that fits through holes in a strap to keep it secure. Before buckles were invented, people had to tie everything with knots or laces, which took much longer and came undone more easily. When you buckle your seatbelt, you're fastening the two ends together.
2. To bend, collapse, or give way under pressure or strain. A bridge might buckle during an earthquake, bending and warping as the ground shakes. A person's knees might buckle when they receive shocking news, suddenly feeling weak and unsteady. When something buckles, it doesn't necessarily break completely: it bends or crumples from stress or force it can't withstand. An old chair might buckle under too much weight, or a student might feel like they're going to buckle under the pressure of too much homework and too many activities.

The phrase buckle down means something different: to focus seriously on work or study, like when you buckle down to finish a difficult project.