Purigen is designed to be a "super carbon" of sorts. Taking out impurities of the water. This can help get clearer water in the way of getting rid of tanins. Can also remove toxins from the water as well. It is rechargeable etc. Known to be compatible with aquatic fertilizers etc. While this isn't required to run a great aquarium, many people feel they wouldn't run a tank without it.
The 100ml size comes in it's own pouch so all you do is add it to your filter.