Priority is a hierarchical bracket of moves that dictates which Pokémon will strike first in battle.
While the comparative Speed stats of Pokémon will usually determine who strikes first in battle, the priority bracket of a Pokémon supersedes it. If a move is subsumed under a higher priority bracket, the user will outspeed the opponent no matter how inferior its Speed is compared to theirs; conversely, Pokémon deploying moves of lower priority brackets will be premoved by their opponent no matter how faster their Speed is. If both Pokémon are deploying a move of the same priority bracket, it the first strike will sheerly boil down to which Pokémon has greater Speed stats or another tie-breaker if applicable.
Priority-bracket values range from +5 to -7. Most moves are lumped in the priority bracket of 0; as such, moves that have any priority-bracket value higher or lower than that are referred to as priority moves. Positive priority brackets are referred to as increased-priority moves, whereas negative ones are known as decreased-priority moves.
Increased-priority moves can be nullified by the deployment of the Quick Guard move.
The priority hierarchy can still be affected by priority-inhibiting items; for example, a Lagging Tail will ensure that the holder cedes the first attack to the opponent regardless of their Speed and the priority bracket of the move used.