Potion is the art of making potions. Some potions allow you to restore your material/ethereal points, and your food level. Other potions can be used for boosting your skill levels or abilities.
When mixing, you do not need to have EMU left over to mix an item; but can have a full EMU of ingredients, and the product will replace ingredients lost, rather than needing the space for the product.
You need Vegetal Nexus to make Potions. The higher the nexus, the better the potions you can make.