Combat Rating is a modifier used when an investigator fights a monster or Ancient One. The number applies to the combat check the investigator must make when attacking a monster. Combat ratings vary from monster to monster with a range from +1 (ex. Cultist) to -6 (ex. Cthulhu).
Joe Diamond chooses to fight a Star Spawn whose combat rating is -3. He successfully passes his horror check and prepares to attack the monster. He totals up his fight skill, which is currently at +8 given his weapons and base skills. Joe must deduct 3 from this number due to the monster's combat rating. Now he can only throw 5 dice.
For more information see: Combat