What is it?

The basic concept of an engine is something that exploits the difference of temperatures, the high and low end, and applies it to work on something. A Stirling engine in the simplest explanation is an external combustion engine. It heat source come from outside the system rather than being pumped into the system. It is similar to a steam engine but doesn't use steam to power it. Instead it cycles air, or some type of gas,  from cold to hot and back again, to power a machine of some sort.