Nests in Pokémon GO an area where a single type of Pokémon spawns in abundance. Most Nests are located in public park locations. You can usually tell if you are in a nest if you see multiple of a single type in the Nearby Pokémon screen.
Sometimes the type of Pokémon in a nest location will shift to a new type! This was first observed on 7/29/16 during a server balance update. One of the first migrations observed was an Onix Nest shifting to Seel Nest.