A lifeguard is a person whose job involves watching over swimmers and rescuing them if they're in trouble. You have to do more than just twirl your whistle to be a lifeguard; you'd better be a very good swimmer as well.
At the beach and the pool, lifeguards wear swimsuits and carry whistles for catching the attention of rule-breakers, like kids who are dunking each other or jumping into the shallow end. Lifeguards sit perched on high chairs, ready to swim to the aid of someone who's drowning or injured. In the seventeenth century, a lifeguard was a "solider's bodyguard," and by the late 1800s the meaning changed to "watcher of bathers."