is this available for all classes?

I have made a quick search about this but it didn't help, i dont see anyone using it in vids on youtube either.
so if someone can tell me about this skill I appreciate it.

I think only a few classes have lock-on type skills. Priest, Mystic and Sorcerer are three I know that have them. Basically, you press the button you have assigned to your lock-on ability, face your screen so your cursor roughly crosses over your target, then press the button for your lock-on ability again to release the spell. There's no targetting in Tera, it's all about facing your camera/reticle at where you want your spells/abilities to land. It takes some getting used to, but makes for a LOT more exciting combat IMO.

Pretty much any ranged class has some kind of lock-on skill.
As mentioned, the clothies (Priest/Sorc/Mystic) all have skills that use lock-on.
Archers also have an early skill that hits multiple targets that uses lock-on too.

I've only tried out 4 classes, but the ones I played all had at least one lock-on ability. (I played an archer, sorc, mystic, and priest.) Archer had a lock-on they could use for multiple target damage; sorc had one for a CC ability and one for damage; and priest and mystic had heals and CC that were lock-on, with mystic also having a lock-on DOT ability.

I don't know about the other classes having lock-on skills, but Arturous went over pretty much how those skills are used and the usual way combat works in TERA.