To unstrap means to unfasten or remove straps that are holding something in place. When you unstrap your bicycle helmet, you're releasing the buckle under your chin so you can take it off. When astronauts unstrap themselves from their seats after landing, they're freeing themselves from the safety harnesses that kept them secure during the flight.

You might unstrap a backpack, unstrap a watch from your wrist, or unstrap luggage from the roof of a car. Parents unstrap toddlers from car seats, and rock climbers unstrap their harnesses after reaching the ground safely.

The opposite action is to strap something, which means to fasten it securely with straps. While strap suggests making something safe and secure, unstrap suggests release and freedom after being held in place.