When you put a cup with a drink inside on a surface, and then take it off. It leaves behind a ring-shaped formation of what you just drank. This is the result of spilling some of the drink over your cup a little bit, so some of the liquid trickles down the the bottom and stays there. Other times, it's because you shook the cup while it is still on the table, thus the liquid collects at the bottom part of the cup. After a while, it will leave a sort of dried up ring, except with the case of water, it usually evaporates. See water ring for cups with water. That dried up mess is what you call a "residue ring." Note that this can also happen with bowls, plates, pans, pots, aluminum cans, circular objects, etc. It is usually found at parties, messy houses, kitchens, and such.

When you eat dirt for the first time and leave your family to build a shack in the woods