Mana burn

Mana burn is loss of life for not using mana in your mana pool when a phase ends. For instance, you tap a Swamp to cast Dark Ritual which gives you File:Manab.gifFile:Manab.gifFile:Manab.gif which you use to cast Erg Raiders. If you can't use the remaining File:Manab.gif then you will take 1 damage from mana burn.

From the Comprehensive Rules:

• 300. General
  • 300.3. When a phase ends (but not a step), any unused mana left in a player’s mana pool is lost. That player loses 1 life for each one mana lost this way. This is called mana burn. Mana burn is loss of life, not damage, so it can’t be prevented or altered by effects that affect damage. This game action doesn’t use the stack. (See rule 406, “Mana Abilities.”)

In-world meaning

In storylines mana burn happens when a wizard holds mana and don't use it to cast the spell. Such wizard can seriously suffer from it, get killed or even be burned to ash if the amount of mana was too big. Some victims of mana burn can be also turned to the Fallen.