Far away, less is enough.

A LOD (level of detail) is a lighter copy of a model that the engine shows when the model is small on screen. Mipmaps do the same for textures. Watch a LOD Group switch, set its thresholds, make the LODs with Decimate, stop a floor from shimmering, and fit a whole scene in a budget.

STAGE

3D Viewport Distance

MMB / LMB drag orbit · Wheel distance

LMBOrbitWheelDistanceSpacePlayCtrlZUndo

KEY IDEAS

Spend where it shows

The screen has a fixed number of pixels. Triangles and texels smaller than a pixel cost time and memory and add nothing you can see.

LOD: Level of detail: LOD0 is the full model, LOD1, LOD2… are lighter copies. The engine swaps them as the object gets smaller on screen.
Screen relative height: How much of the screen height the object fills (%). A LOD Group switches on this number, not on the distance, so it works for any object size and any screen.
Error in pixels: How far the lighter mesh is from LOD0, seen on screen. Below about one pixel, nobody can see the change.
Popping: The visible jump when a LOD switches too early. Good thresholds and Cross Fade hide it.
Mipmaps: Smaller copies of a texture (½, ¼, ⅛…). The GPU reads the one whose texels match the screen pixels: log₂(texels per pixel). They cost +⅓ memory.
Filtering: Bilinear reads one mip, Trilinear blends two, Anisotropic keeps floors sharp at grazing angles.

BACK IN BLENDER AND THE ENGINE

LODs in practice

Blender makes the meshes; the game engine switches them. Mipmaps are made by the engine when it imports a texture.

Duplicate the model (Shift D) and name the copies _LOD0, _LOD1, _LOD2… Unity and Unreal read these suffixes on import.
Add a Decimate modifier to each copy. Collapse with a Ratio (often about half the triangles for each LOD); watch the Face Count. Un-Subdivide for grids, Planar for flat hard-surface parts.
Check the silhouette from the distance where each LOD will start, and keep UV seams and hard edges (Planar › Delimit).
Export all the LODs in one FBX. In Unity the LOD Group sets the thresholds in % of screen height and the Fade Mode; in Unreal the LOD Settings use Screen Size.
Textures: keep Generate Mip Maps on for 3D textures, use Trilinear and Aniso Level for floors; turn mips off for UI drawn pixel for pixel.
Use LOD Bias in the quality settings to trade detail for speed on weaker machines.