How does a buoy work?

A buoy is designed to float and be visible. They are normally anchored to the bottom, to act as a navigation aid in a specified position.
Most navigation buoys have a flashing light on them, with a distictive pattern, which identifies their position and purpose.
The lights, these days, are LEDs, controlled by an electronic timer, powered by a battery, which is charged via solar panels. Previous designs used gas lamps and mechanical timers, which needed more maintenance.