The longer version
A skill rating is a single number per player per sport. It starts from a self-assessment or a coach’s placement and then moves with results: beat a stronger player and it rises, lose to a weaker one and it falls. Most systems use a scale such as 1.0 to 7.0 or 1 to 10.
Clubs care because mismatched games are the main reason a player does not come back. A rating lets the desk fill open play, seed a box league, and suggest partners without knowing every member personally.
Ratings drift when few results are recorded, so the first mistake is relying on self-assessment alone. The second is showing a precise decimal to players when the club has only recorded 3 matches. Treat early ratings as provisional and say so.