App was rejected for use of protected 3rd party material
There are several avenues you can take to resolve this. The first is to log into the developer center for your iOS developer account and go to the Resolution Center and verify that the reason is correct and see if they have identified specific assets (music, artwork, code that could reasonably be protected by copyright in the US.).

There is an App Review board, but I wouldn't rush to that as it's kind of a final step and you'll want all your ducks in a row if you go that route.

If you really have no clue what parts of your app might be problematic, you can use a support incident and ask Developer Technical Support to point you to the specific items that are problematic and flagged by the reviewer. My guess is you will have a good idea and can substitute a royalty free drawing or song to resubmit the app and confirm whether the items you suspect being flagged for rejection are the actual items.

At that point, you will need to decide if you want to push things and use the assets and somehow justify why copyright material isn't in your app or get approval from the copyright owner and submit that evidence to Apple.

You could also ask the reviewer to send you the list of assets so you can start the rights check documentation. Depending on the issue they see, it could be a simple letter from your lawyer to something much more involved if Apple has had previous legal requests from copyright holders and you have run afoul of that with your recent review denial.

All of the above would be most applicable if the app contains assets that were flagged. If your app during testing consumes or downloads other copyright material, you may need to have a more direct discussion with Apple to understand if you are even going to be allowed to distribute your App through Apple's storefront.

Worst case, you can request clarification of the rejection from https://developer.apple.com/appstore/contact/