An auto top off system usually impments a float switch as has been described. The float switch can either be mechanical (simular to the float in a toilet tank) or electric. If its mechanical then as the water level lowers it opens up to allow water to gravity flow into the system. When the water rises to a specific point the float cuts the flow off.
In an electircal switch a simular float is in place. When the float drops to a specific level it opens a relay and that completes an electrical circit that powers a small pump. The pump then puts water in the sump slowly via a hose. As the water level rises the float rises till it eventually closes the relay and that in effect cuts power to the pump.
The use of a float switch allows your tank to remain as consistant water level as possible. If you use a gravity feed system then weekly you would replentish a bucket or some other container that held your replentishment water. If you used an electrical system you could use a much larger container like a 30 gal
trashcan OR if you got real fancy you could connect the float switch up to a RO
unit and in essence you would never have to add more water to your system except for when you did a water change.