I have heard about game engines and how you can find the source code for the game engines of some older games, but I do not know the definition of what a game engine is? Could someone please explain this?

engines is a basic terminology describing code that performs a specific set of functions. All games have atleast a game engine, most also have graphic engines, sound engines etc.
A game engine basically is the code for the in game functionality itself. A graphics engine would be in charge of displaying the game, a sound engine would play music and sounds.

Somebody asks this question about once a week, please use the Search tool.

Invader X
Invader''s Realm