Original: Heat is transferred from the source, through the "working body" of the engine, to the sink, and in this process some of the heat is converted into work by exploiting the properties of a working substance (usually a gas or liquid).
Simplified: Heat is transferred from the source, through the "working body" of the engine, to the "sink", and in this process some of the heat changes into work by using the qualities of the gas or liquid inside the engine.