What is a PTO on a riding lawn mower?

PTO is short for Power takeoff. It is to control a shaft comming out of a vehicle to power things like a snow blower attached to the front, or to control things on the back of a tractor... I never heard of a PTO on a lawn mower... It must be a pretty big lawn Mower.

PTO on a riding mower will be an electric engaged clutch used to power a belt drive or other attachments.

Oh yes, your right.. perhaps it may be the pull handle to engage it also.