Potions are a series of consumable items that can only be used once. Potions can be found in the shop during the resting period between two bosses. The potions start from cheap like , , or which only give health or mana, till more expensive potions like, , , .
You should usually at least buy, depending on the difficulty, one or two, and one before each boss fight because you cannot solely rely on priest's heals and you will eventually run out of mana.
Each player has only two slot for placing their potions in but you can stack potions of same kind in the same slot. Of course, there is a cap on how many of the same potion you can carry and some of them come in stacks of two or more.