Mana burn is an obsolete game concept in which a player would experience loss of life points for having excess mana in his or her mana pool when a phase ends. For instance, if a player tapped a Swamp to cast Dark Ritual, which would give that player in his or her mana pool, then cast Erg Raiders (which costs ), and then let the phase end in which Erg Raiders was cast but did not spend the remaining , that player would lose 1 life from mana burn.
 In-world meaning
In storylines mana burn happens when a wizard holds mana and doesn't use it to cast the spell. Such a wizard can seriously suffer from it, be killed, or even be burned to ash if the amount of mana was too big. Some victims of mana burn can be also be turned into the Fallen.