Dwellings are a combined upgrade for Apple Trees and Bathtubs that can be unlocked in the second part of the game: they feed and clean a small number of Thronglets in one go. You can also unlock Fancy Dwellings for a greater Thronglet feeding/cleaning capacity.  Dwellings are unlocked at 200 gems, and Fancy Dwellings at 200,000 gems. Both buildings are unlocked via the Market.