The player’s RC rating is based on their respective talents and matchup. This indicates a players matchup and whether it is better or worse than their original rating. For example, if Joe Schmo has a 83.8 rating, it means he has a great benefit in today’s matchup against the opposing teams pitcher.
The differential is a player’s rating today compared to the rest of the season. If it’s low, that indicates he has a poor matchup. In addition, if that is high that player has a GREAT matchup.
A way to use the differential is to avoid players. For example, if Paul Goldschmidt has a -4.2 difference rating against Corey Kluber, meaning this matchup is extremely tough for him. This makes sense given that Goldschmidt’s success has come against left-handed pitching.