It's not a difficult concept. Wikipedia has a good explanation.
"A game engine is a software development environment designed for people to build video games. Developers use them to create games for consoles, mobile devices, and personal computers. The core functionality typically provided by a game engine includes a rendering engine ("renderer") for 2D or 3D graphics, a physics engine or collision detection (and collision response), sound, scripting, animation, artificial intelligence, networking, streaming, memory management, threading, localization support, scene graph, and may include video support for cinematics. The process of game development is often economized, in large part, by reusing/adapting the same game engine to create different games[1] or to make it easier to port games to multiple platforms."
Some of the bugs in Bethesda's games are likely caused by issues with the game engine. Other bugs would be there regardless of their choice of engine.
