A Plate is a held item that raises the power of the holder's corresponding-type moves by 20%. When held by Arceus, its typing and its signature move Judgment become the type of the Plate, and any corresponding-type moves, including Judgment itself, receive the power boost.
They can be obtained as a drop from Mega boss Pokémon.
List of Plates
Trivia
The Pixie Plate used to be a tier 2 special drop, and had a crafting recipe as well. This recipe no longer works.