Level Of Detail (or LODs for short) in game engine language refers to the currently visible detail of any *mesh object.

*Remember for our purposes, our parametric CAD data is now polygon mesh geometry!

In the most uncomplicated terms, geometry that is closest to the camera (or game character) displays the highest amount of detail. As the object becomes farther away, the rendering of the geometry cycles through other LODs each with more simplified geometry.

While you can customize your LODs to suit your project needs, the most common types are preset in the Unreal Engine and selectable as:

-Small Prop
-Large Prop
-Deco (for decorations)
-Vista
-Foliage
-High Detail

Here's the link to learn all about LODs direct from the Unreal Engine documentation:
https://docs.unrealengine.com/en-US/Engine/Content/Types/StaticMeshes/HowTo/LODs/index.html

And here's a link for setting up Automatic LOG Generation:
https://docs.unrealengine.com/en-US/Engine/Content/Types/StaticMeshes/HowTo/AutomaticLODGeneration/index.html