LODs refer to simplified versions of the original mesh. So they are not flat like billboards, and still have the "depth" to them. Think for example, a pine tree

LODs refer to simplified versions of the original mesh. So they are not flat like billboards, and still have the "depth" to them. Think for example, a pine tree with many shapes for branches in the foreground, but in the distant background the entire body of the leaves can just be a simple cone.