It simply indicates you're running a pre-release build that's not guaranteed for production use. Rest assured, there's no issue with your license. To remove the watermark, you need to opt-out of the Insider Program. Go to Settings, click on Windows Update and select the Windows Insider Program.
What does evaluation copy mean?
evaluation copy. plural. evaluation copies. DEFINITIONS1. a piece of software that has the most important features of a new software product and is designed for people to try before buying the full product.