The original RTM version of Windows 10 was released on July 29 back in 2015. Since then, Microsoft has released 3 major updates for the Windows 10 platform, including the recently released Creators Update (version 1703). At the same time, the original Windows 10 has received a bunch of cumulative updates, including security fixes and stability improvements. However, Microsoft is now supporting its frequently updated products for 18 months or 1.5 years, and the time for Windows 10 ver. 1507 support to end is almost up.
If you're still running the original version of Windows 10 that was introduced back in July 2015 (version 1507) and for some reason still ignoring all of the offered major updates for it, Microsoft is giving you two extra months to decide if you want to get future patches and security updates. The updated end-of-life date now states 'May 2017' compared to the previous March 26 schedule. All machines running Windows 10 1507 (build 10240) will continue to get updates during that period.