What is Centering?

You're triggering Centering. This is gained at level 10 as a Jedi Sentinel (so it is Sentinel specific, though Sith Marauders have an equivalent ability).

You gain stacks of Centering whenever you spend focus or kill an enemy, and it doesn't do anything on its own until you reach 30 stacks of the buff. At that point, you can trigger Zen, Transcendence, or Inspiration:

• Zen - learned at level 10. Gives a bonus depending on your current lightsaber form.
• Transcendence - learned at level 22. Temporary Movement and Ranged Defense Bonuses for you and your party.
• Inspiration - learned at level 44. Gives a % damage bonus to you and your party.

A Jedi Sentinel gains the ability to Center. When a Sentinel attacks with force, stacks of centering are added. With more centering stacking come the ability to use advanced levels of fighting stances Zen, Transcendence, and Inspiration. For example, the activation of Zen takes place with 30 stacks of Centering. The Zen activation and specific lightsaber forms add different abilities to the Jedi Sentinel. The Zen activation and the Juyo form increase the critical chance of burning melee attacks by 100% and heals the party through the burn damage on the enemies.

More information here.