Concentration represents a character’s ability to focus on a given task for an extended period or during times of stress or distraction.
The most common usage of this skill is maintaining spells that have a duration of Concentration.
Normally, Concentrating on a spell is a Minor Action and the DC to maintain a spell is 11.
However, whenever a spellcaster maintaining a spell takes damage, they must succeed in a Concentration check with a DC equal to the damage taken.
Failure on such a Concentration check means that the spell is no longer being concentrated on. If the spell has a Duration of Concentration, it ends immediately. If it has a duration of Concentration + x rounds, it ends in x rounds.