An option for developers and business people who like to test new software. Introduced prior to the release of Windows 10 in 2015, the Insider Program delivers members a beta version in several stages before the final release date, and the feedback is used to fix or improve the upcoming release.
Fast - Slow - Preview Rings
Builds of Windows are released in rings. The Fast ring is the first time insiders see the version and receive releases from one to two times per week. The build is then released to thousands of Microsoft employees. After the Fast and Microsoft rings, insiders see the product again in the Slow ring. Finally, there is a Release Preview prior to the actual release of the version. See Windows as a Service