Farm tractors and some trucks and industrial engines can have a PTO or Power-Take-Off shaft that is used to power attachments such as sprayers, hydraulic pumps and, well, generators.
The PTO is low speed, so a PTO generator usually has a gearbox to step the speed up to the RPM needed to turn a generator. PTO's are pretty standardized and use a short driveshaft to couple the tractors PTO to the attachment.
PTO generators are used where remote power is needed and a PTO-equipped vehicle is available. It saves having to buy and maintain another engine.