Forage has no effect in battle.
Outside of battle
After winning a battle, any Pokémon on the team with Forage that is not already holding an item has a 10% chance of creating a held berry item for itself. This occurs even if the bearer of Forage didn't participate in the battle, and even if it is fainted.
Pokémon with Forage can obtain the following berries (NB. these are still being decided on):
Although Musburry changes its appearance depending on which berry it is holding, this is not due to Forage.
Pokémon with Forage
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