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
Then you remain at the ranking you were at before you got injured. So if you were Upper Tier 3 when you got injured, you will keep that ranking until you come back and get assessed.