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.”)