From Bulbapedia, the community-driven Pokémon encyclopedia.
Pester Balls (Japanese: イヤイヤボール Reluctant Ball) are multi-colored balls that at first appear to be regular Poké Balls. However, instead of catching Pokémon, they release a noxious Pokémon repellent that can knock out or stun Pokémon.
In Pokémon Snap, Professor Oak gives Pester Balls to Todd Snap after he obtains a total score of 75,000 points for all of the pictures in the Pokémon Report. They help him flush out wild Pokémon, provoke an irritated reaction, and sometimes even knock them out of the way.
Pester Balls have never been used in any other game, though they may be made from the same chemical found in Repel.