It's an additive for the water that supposedly calms the fish and increases O2
. They are never needed in a well maintained tank (or IMO
Slightly off-topic: Bettas need more than a bowl (at least 2.5G) that is filtered and heated. Any shop that tells you "they're fine in a bowl" or "they only grow as big as their enclosure" should be avoided.
Welcome to AA by the way!