Level of detail, or LOD, refers to how objects are rendered at a distance in-game. As the distance between the player and an object increases, the object’s visual fidelity will decrease and sometimes vanish altogether (depending on the importance of the object).
“Pop-in” is an effect where objects or foliage may be invisible until the player is close, causing a visible and noticeably irritating “popping into existence” of the object. The best way to test this is to walk around Whiterun hold, one of the most open areas in the game. Pay attention to the landscape around you. You will notice trees and grass fading in and out, shacks and other objects fading in and out of quality.
If you have any mod-added objects, they might not come with pre-generated LODs, causing them to pop in out of nowhere when you are close enough for the full object to be rendered.
To make our own LODs, we use a program called DynDOLOD. You will need:
Install DynDOLOD and TexGen as programs into your mod manager, and the resources as a normal mod.
Launch TexGen, set your settings, run it, add the output to MO2. Then do the same with DynDOLOD. Please watch the video to see these steps, it looks overwhelming but is actually quite simple. These will give you basic LODs, and does not cover terrain or grass.
Check here for out advanced LOD guide.