Usually blue in appearance to contrast with the red color of health potions, mana potions restore a character's lost mana points (or their equivalent in the given game) when used. In most games there are several types of mana potions that restore differing amounts of mana. Some games also have a variant of the mana potion that affects the player's entire party rather than a single character.
In the Diablo series there exist potions that restore both health and mana over time rather than all at once. Most classes do not regain mana automatically, so mana potions are the only way to build magic power back up again, unlike with health points which may be regained by other methods.