Welcome to Android Central! That indicates that the device was managed by a school or company. You actually can't get rid of that yourself -- it has to be disenrolled by the organization that was managing it. If the seller didn't disclose this fact, I would bring it back for a refund.
A managed Chromebook's usage can be monitored by the organization that administers it, and its functionality can also be limited. If this device is from a defunct company, or was retired by that organization, then it's unlikely that anyone would be around to monitor it -- but you will still run into issues with limited functionality (and perhaps not even be able to sign in with your own personal account).