A tier list is a catalog of characters in the BlazBlue fighting game series, ranked based on their general efficacy in competitive play. The lower the character is on the tier list, the more difficult it is to win a match playing as him.
A character's placement on the tier list is determined by their power, speed, moveset (the attacks a character can use) and how well he can be played when matched up against another character. Iron Tager is considered to be a low-tiered character, for example, due to his slow speed and unfavorable matchups against other most fighters.
The fighting game community creates tier lists by researching each fighter through competition and online play, and a consensus for each character's place on a tier list is reached unofficially via online voting or forum discussions. A tier list is constantly changing and morphing, based on how often a character is used or the changes made to the character by the game's development team.
Tier lists are guidelines for selecting a character when playing competitively. They can also be referenced as an analysis of how a fighter moves and attacks and for how well a fighter matches your playing style.