LOD means Level of Detail. It is a way of rendering things that are less important in a lower detail. The common case is based the less important part on the distance. For an object that is far away all the triangles will not be visible anyway.
With model LOD one swap out the high-resolution model with lower-resolution models at far distance from the observer. The further away things are the smaller the get on the screen from perspective so details are harder to notice. There are two ways the lower-resolution models can be created, Discrete or Continuous LOD.
Discrete LOD (DLOD)
Continuous LOD (CLOD)
Hierarchical LOD (HLOD)