Some older Windows Insider Builds that are out in the wild will begin hitting a timebomb for expiration beginning today, September 15. Users on those devices will get a daily expiration notification on their systems.
After October 1st those systems will reboot every three hours. Two weeks later, on October 15, they'll stop booting all together.
In order to avoid any issues you need to upgrade those Fast Ring systems to the latest build, such as 14926, which expires on May 1, 2017.
If you are in the Slow or Release Preview Ring on Build 14393 this does not apply to you but you will eventually receive a build expiration notice.
Be sure to check out all of our Windows 10 Anniversary Update Coverage.