TrueSkill is a ranking system developed by Microsoft Research to evaluate the skill levels of players in competitive games. It uses a Bayesian approach to estimate a player's skill based on their performance in matches, taking into account both wins and losses. This allows for more accurate rankings, especially in games with multiple players or teams. The system assigns each player a skill rating and an uncertainty value, which reflects how confident the system is in that rating. As players compete, their ratings are updated, allowing for dynamic adjustments that better represent their current abilities over time.