It is a floating buoy that is used to support their victims in the water and also a signalling device.
It is always kept with them at all times, either on a hook above at every lifeguard tower or carried with them when they are patrolling the beach.
There are also many kept in their jeep.
When they see a victim in trouble they rush to the scene quickly unwinding the rope, placing the black strap of it over their shoulder and dive into the water towards the victim.
The can is then placed under that person's arms while they are held securely by the lifeguard as they gently bring them back to the beach.
During a rescue, the lifeguard sometimes raises their can in the water. Raising it vertically means the lifeguard is signalling to lifeguards onshore that he or she needs more manpower to assist with the rescue. Raising it to a horizontal position means that a rescue boat is required to assist with the rescue. They also wave their rescue cans as they patrol up and down the beach. This is so that the lifeguards working on both sides of them can easily spot them in case backup is needed.