PokeStop

It's basically a spot where a specific type of mon regularly spawns. The most recognizable, classic nest is a cluster spawn -- where 3-4 mons will spawn around you. So, 3-4 charmander or dratini. The other example is where a place -- like a park -- regularly spawns, say, jigglypuffs throughout. The latter example is tricky because people confuse them with biomes (like dratini spawning in water biomes where you find magikarp, psyducks & slowpokes) but that's the basic idea.