Rooibos tea adds tannins to the water, which colour it brownish and help the fish's slime coat and immune system. They also make the water a bit more acidic, which bettas like. You can also use Indian Almond Leaves (IAL) or dried oak leaves. You can buy IAL on ebay.
I found plain organic rooibos in my local chain grocery store, they were the store brand. Soak the tea bag in the water until it turns a medium brown colour, then remove it.
Note: If you add leaves to the water, make sure you add them to fresh water when doing a water change.