The Pokémon Watch (Japanese: ポケモンウォッチ Pokémon Watch), or Pokétch (Japanese: ポケッチ Pokétch) for short, is an electronic device found in Sinnoh. It was manufactured by the Pokétch Company and appears to be a watch with a touch screen.
TL;DR, a Poketch is basically a Sinnohian(?) PokéGear. It is also similar to the C-Gear from Unova. It can be used to do various things such as taking memos, tracking down roaming beasts, finding hidden items etc. Every once in a while, you can get a new Pokétch app installed for your Pokétch by revisiting the Pokétch building. You can also get some outside of the building, such as the Friendship Checker from a woman in Eterna City's Pokémon Center.
Hope it helps!
Source (Pokétch)
