This means you place thousands of vegetation models and they will be merged together to a few one.
A example: if I place 4000 grass models into a level (60k polygons), the framerate drops down to 15 fps on my pc, because of the huge amount of draw call. A function creates a single model out of the 4000 entities. So the engine have to do only one draw call. After that the framerate is over 500 fps. If all grass models in the level exceeds a poly count of 65536 polys, they have to merged into several models.