Concentration is a defensive attribute, and affects a character’s ability to resist being interrupted while casting spells. Concentration reduces the likelihood that incoming attacks will interrupt a spell while it is being cast. The chance that the spell will not be interrupted increases based on the amount of Concentration the character possesses.
Caster classes like Clerics, Chanters, Sorcerers, and Spiritmasters benefit from Concentration, while other classes receive little to no benefit.
Concentration is perhaps most important for Clerics, who tend to be the main healers in Groups and Alliances, and can’t afford to have their casts interrupted. However, all caster classes can benefit from increased Concentration in both PvE and PvP situations.