When you enter a dungeon, you have three potion slots. When you tap a potion slot, your potion inventory will open and you'll be able to see all the potions you have. Select the potion type you want to take into the dungeon with you. For some potions like the Healing Potion, you can have one per slot. However for other potions like Le Water Potion, you can have three per slot. You can only select a potion once, so you can't have the same type of potion in two different slots.

In the dungeon you can use the potions at the beginning of your turn before you draw any cards. Select the potion you want to use by tapping it. If it's a potion that affects only one hero, like a Heat Potion which removes freeze, you must tap on the hero you want to use the potion on. If a potion can't be used, for example if you try to use a healing potion when your health is already full, or you try to remove poison when you are not poisoned, the potion will not be consumed.

You can obtain potions from chests.

Potions:

Healing Potions increase your health
Star Potions give you skill energy
Antidotes remove Poison
Le Water Potions remove Burn
Clarify removes Corrupt
Heat Potions remove Freeze
Enchanted Water removes Silence
Stimulants removes Stun
Purify removes negative status effects e.g. when a monster casts a spell that lowers your DEF or REC
Steelskin potions increase your defense so enemies do less damage
Strength potions increase your attack