Something inflatable can be blown up or filled with air. A balloon is inflatable — you can either blow it up, or fill it with helium so that it floats.
Car tires are inflatable, and so are beach balls, bicycle inner tubes, floating pool toys, bouncy castles, air mattresses, and some lightweight boats. If you're in a car accident, the inflatable air bag will fill quickly to protect you from hitting the dashboard. The adjective inflatable comes from the Latin inflare, "to blow into or puff up."