A Mirror Herb is a held item that copies all of the opponent's increased stats. It will be consumed after use.
Breaking a Mirror Herb crop drops 1 Mirror Herb.
A Mirror Herb copies all of the opponent's increased stats. For example, if the opponent uses Swords Dance, the Mirror Herb will activate and raise the holder's attack stage to +2 as well.
A Mirror Herb does not allow the holder to learn an egg move when in the party with a compatible Pokémon outside of battle currently (as of version 1.4.0).