The longer version
A skill band groups a range of ratings under a label a player can read: beginner, 3.0 to 3.5, intermediate, advanced. Bands do the work of ratings without asking players to care about decimals.
Clubs use bands to gate sessions. An open-play session or a class is tagged with a band, and players outside it either cannot book or see a warning. This keeps a 2.5 beginner off a 4.0 court and makes the session worth coming back to.
Bands that are too wide produce lopsided games. Bands that are too narrow leave sessions half empty. Most clubs start with 3 or 4 bands per sport and split the busiest one once it fills consistently.