Recent performance. This has nothing to do with what you have done in your career as a whole, but about how you are playing right now.
The lowest introductory Tier is Tier 3 and the highest is Chief Oga, that can only be occupied by one player.
Within each tier there is a Level 1,2 and 3. So once you are a Tier 2 Level 3 for example, that means you are close to becoming Upper Tier 2 Level 1
Those who are missing are unranked and are not yet good enough to earn a ranking.