Why is my toilet leaking underneath?

This is caused by a flush valve that is not closing completely when the tank is full. The flush valve closes when the large float in the tank rises to the top as the water fills the tank. The float is often at the end of a soft metal rod about a foot long. Bend this rod downward just a bit and the valve should shut off properly.

If the float is a sliding type found on many newer low-flush toilets, I have no suggestion other than that something might be caught in the assembly, or perhaps you will need to replace the entire flush valve.