The standard water heater in most homes is what’s called a storage tank water heater. This is the familiar water heater with a large tank that uses either gas jets below the tank or electric heating elements inside it to heat up the water. Because the water in the tank gradually cools down as heat escapes to outside the tank (this is called standby heat loss), the gas jets or heating elements must come on periodically to restore the temperature, even if there’s been no demand for hot water.
You should be accustomed to the pattern of when your water heater turns on. If it starts to cycle on and off frequently when there’s no demand on the hot water, it may be trouble with the water heater. But it’s also possible that it comes from a second, unexpected source: a plumbing leak, maybe even a major slab leak.