It's Microsoft's attempt at making a ranking system similar to
Glicko (2), but with direct support for team-based games. If you're thinking about using it I would strongly recommend Glicko instead. Glicko can probably be used as a team-based ranking system by combining enemy and teammate rank distributions into the single enemy rank distribution without complicating things too much. Trueskill on the other hand uses some obscure factor graphs with message passing, which you need to understand to modify those to work with different conditions than just win/lose. Maybe someone better than me at that area of math can explain if/why this is better, but even with my limited understanding I don't see it offering a solution to the complexities of team-based rankings more than a modified Glicko system. Trueskill is also patented by Microsoft, and I suspect that is moreso the reason for them making a new ranking system than just modifying Glicko to suit their needs (like
Valve did with CS:GO).
