Maybe your child has started to go to tournaments. You've probably noticed a rating number next to some of the competitors names. You may have encountered ratings at registration to a tournament that features both "rated" and "unrated" sections.
Ratings are used in chess to calculate an estimate of the strength of the player by comparing their performance to the other players. To most players what the number represents about them is more important than how it was calculated. At the scholastic level, for instance, the NWSRS starts young players at 400. An average rating after gaining some skill and competing may be around 600-800 while scores over 1000 are more exceptional. The top level players may have ratings closer to 2000.
Scholastic players in Washington State and Oregon need only concern themselves with two rating systems. The most important is the NWSRS which is a system specific to scholastic tournaments here in the Pacific Northwest. At a national scale, and primarily for adults, the USCF ratings are used. Learn more about NWSRS ratings, including your child's rating at this site: Northwest Scholastic Ratings. USCF Ratings can be learned about on this informative wikipedia page.
Partly lending to the mystery and confusion surrounding ratings is the somewhat complex details and mathematics behind them. Calculating a chess rating involves a specific algorithm which initially takes into considerations facts such as the age of the player, if they have any previous ratings and how many games they've played overall. The initial rating is considered provisional until the player has at least 8 wins and losses combined.
If you're mathematically inclined you can learn more at The Rating Algorithm