Bjorn asks you to bring him some Cheri Berries, which you can get by throwing a partner pokémon at a berry tree.

The Request

Talk to Bjorn in the Deertrack Path area of Obsidian Fieldlands to accept his request.

Get Cheri Berries

Find a berry tree in Obsidian Fieldlands with red berries in it, and throw a partner pokémon at it. The pokémon will knock down the berries and they will go into your satchel.

Hand Over the Berries

Give Bjorn 3 Cheri Berries. You will get 15 Poké Balls.