The difference between armor and rating

Bound items are a feature implemented in MMORPGs to avoid abuse of twinking.

Bound items are designed to prevent them from being shared with other characters, to avoid easily making overpowered lower level characters. If you have a bound item, it can (generally) be sold to in-game vendors, but it cannot be traded to another player.

Unbound items, however, can be freely traded from player to player, and can be used by players who aren't the ones who initially obtained the item (whether from quest, random drop, or by purchasing it from a vendor), assuming that the player meets all of the other requirements for the item.

Bound and unbound is not random. Quest items are typically bound, as they are the reward for that character completing the quest, and therefore it is expected that that character will be the one to use it. The same applies for specific reward drops from difficult encounters, such as raid or group instances.

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.