Inventory

While fighting, pressing and holding will open the quick inventory. Once open, press the arrow corresponding to item you want to use, then release start to use the item.

Healing items will do as advertised, they will restore your health/magic and remove bad status effects like poison or curse. They have a limit of how many you can carry though.

On easy mode it's 9 potions, on normal it's 7, on hard it's 5, on nightmare only 3.

Rosary is a special case, you can carry only one of those.

You start the game with 1 potion of each kind and no rosary. On continue, if you used up your entire stock of specific potion type you'll get one potion of the same type for free.