Windows 10 version 1511 was released in November 2015. Since then, Microsoft has released a few major updates for the Windows 10 platform, including the recently released Creators Update (version 1703). At the same time, previous Windows 10 versions have 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. 1511 support to end is almost up.
The final date will be October 10, 2017 to allow enterprise clients to migrate their devices in time. This being said, all 4 consumer SKUs of Windows 10 (Home, Pro, Enterprise and Education) will no longer receive any updates after October 10, 2017. Users will have to upgrade to one of the newer Windows 10 versions to continue receiving new bug fixes and improvements.
Windows as a service is an ongoing process: New releases are deployed while old releases are retired. Today, many customers are broadly deploying Windows 10 1703 after successful pilot deployments. And many other customers are in the process of updating the last of their Windows 10 1511 PCs in preparation for the upcoming October 10th, 2017, end of servicing date.
Running the older version of Windows 10 after that date may allow hackers to potentially execute harmful code on your devices via newly discovered yet unpatched security holes. So if you're concerned about your security, don't forget to upgrade to the latest version of Windows 10 before October 10, 2017.