Is this it? If so, it's a heal ball. You can buy them in some Pokemarts. What it does, is if you have like, three pokemon in your party, and, you weaken one and catch it, then, it will be fully healed after you catch it with that ball, and is ready to battle. So it is helpful in places such as the victory road, eterna forest, etc. Where you can't just fly to a pokemon center.