The answer is sediment. Sediment is small bits of dirt, rock and other debris that can work its way into your water heater over time. Being heavier than water, it will settle to the bottom of the tank and build up. It’s not an earth-shattering problem, but in areas having water with a high mineral content, or water coming from a well or other supply that may not be well filtered, it certainly is possible to accumulate a fair amount of material in the bottom of the tank.
Besides reducing the capacity of your water tank, the result is a less efficient water heater. It will burn more gas to get the water temperature to your desired set point and this will reduce the heater's usable life.
A yearly complete flush or power flush should be performed.
JP @ Home Watcher LV