What is a PTO on a tractor?

PTO stands for Power Take Off. It's the means by which a tractor, or truck, or train locomotive can power something other than itself. Like Balers, mower decks, snow blowers, generators, log splitters, etc. Some older PTOs will only work when the machine is stationary. Sort of an eithor/or situation. Most of them nowadays work when the prime machine is moving or not. When you engage the blades on your lawn tractor, you are engaging the PTO.