It's the Explanation Of Benefits. IT is a disclosure of treatment and charges, the date it occurred, etc.

The purpose is for you to verify what the medical provider is saying they did, how much it cost, whether insurance covered it or denied the claim, how much your insurance paid, and if you have a copay or deductible that will be listed as well as the remaining amount (a bill would then be sent separately if that's the case).

It also has a secondary purpose of advising you if anyone fraudulently is using your information to receive treatment. Since you would receive an EOB without having gone to the doctor obviously.