Matchmaking rating, or league rating, is represented by a number that changes depending on the outcome of battles played. In terms of the league system, it determines a league a player is in and helps the system place players with approximately the same rating in relevant leagues. Besides, it just shows how good a Commander performs in battles.
Player's rating is displayed on the profile's screen.
It might be missing if a player has not yet finished a series of league qualification battles of the current season.