Levels of Detail (LODs) let you associate multiple versions of the same geometry to be swapped in and out of the engine based on a certain threshold value.

This lets you produce a high-quality version of a model for up close and a low-quality version for far away, with multiple versions in between.

At any given time, the LOD group makes it easy to display the appropriate version of the geometry (one child of the level of detail group), depending on the group's percentage of screen height.

You can find a video tutorial on setting up LODs here.