Repels are items that exist in three different varieties: Repel; Super Repel; Max Repel. When right-clicked, Repels will keep strong aggressive Pokémon from challenging you for 5, 15 and 30 minutes respectively. When used multiple times, the duration is simply increased.
Repel will prevent any Pokémon from spawning nearby while it is in effect (but only with the Better Spawner).
They can be obtained as drops from uncommon and rare boss Pokémon.