Microsoft Windows Insider Program allows interested Windows fans, enthusiasts, professionals, developers and even Enterprise users to test the upcoming features and updates of Windows 10 before they are released. This allows Microsoft to get feedback about bugs, UI issues, problems with new features, and compatibility issues in order to fix them before the preview version is released to everyone. The Windows 10 Insider Program is optional, but any user with an existing licensed Windows 10 PC can join and start testing out the latest features being developed by Microsoft.
Source: https://www.bleepingcomputer.com/news/microsoft/windows-10-insider-program-what-you-need-to-get-started/