To unbuckle means to unfasten or release a buckle, which is the clasp or fastening device that holds two ends of a strap or belt together. When you unbuckle your seatbelt after a car ride, you press the release button so the belt comes loose. When you unbuckle your shoes or your backpack, you're opening the metal or plastic fastener that was holding them closed.

While buckling something secures it in place for safety or to keep it closed, unbuckling sets it free. You buckle up before a roller coaster starts, then unbuckle when the ride ends. A knight in the Middle Ages would need help unbuckling his heavy armor after battle, since those buckles kept all that metal plating attached to his body.

People sometimes use unbuckle more loosely to mean relaxing after being constrained. After a long, formal dinner, someone might say they're ready to unbuckle and get comfortable, meaning they want to loosen their belt and relax.