Concentration is a Cabal Online skill learned by Warrior, Blader from class rank 2. Description as it appears in the game:
Concentrate one's mind to increase hit rate
Classes that learn it
Other Warrior skills at rank 2
These unlock at the same class rank, so the same quest gives you all of them: Heavy Slash, Press Impact, Magic Blast, Stone Blast, Aqua Blast, Wind Blast, Fire Blast, Ice Blast, Lightning Blast, Dash, Reflex, Sharp Eyes.
FAQ
What does Concentration do in Cabal Online?
Concentrate one's mind to increase hit rate
Which class learns Concentration?
Concentration is learned by Warrior, Blader. It unlocks at class rank 2.
How do I raise my class rank?
Through the class rank quests the game offers every few levels. Each rank opens new skills and, later on, the Battle Modes.