Overlap Group and "Gravity"

I am a newbie... When I touch the + to add an item, one of the choices is Overlap Group. Right in there with this name is a description. And that description reads (in part) "supports gravity". Yet I can find no properties whatsoever related to gravity. So I am confused and do not know what this means. I can see that items placed within the group can be anchored to various locations inside the group. But is anchoring the same as gravity?