Windows 10 v1511 was the first major system upgrade for Windows 10, and was released in 2015 when it was simply called the November Update. Given that this build of Windows 10 is nearly two years old, Microsoft says that it can no longer support it, meaning that you won't receive new security updates or "quality updates".
To find out what version of Windows 10 you’re currently running, type “winver” into the search box from the taskbar (without the quotes).
My current build of Windows 10 is up-to-date, but yours might not be so lucky
Of course, this isn't the end of the world, as you can always upgrade to a newer build of Windows 10 like the Anniversary Update (v1607) or the Creators Update (v1703). And on October 17th, Microsoft will release the Windows 10 Fall Creators Update.
We should note that while you can continue to use Windows 10 v1511, Microsoft strongly advises against staying on this older build:
Since you won’t continue to receive new security updates or other quality updates, it could become more vulnerable to security risks and viruses. For this reason, we strongly recommend that you update to the latest version of Windows 10, as described above, and that you keep up to date thereafter. Microsoft offers complimentary support to ensure your device has the latest updates installed and requires that your device be up to date before assisting with other technical support issues.
Unless there's some weird software incompatibility that is preventing you to upgrade to a newer build of Windows 10, there's really no reason to stay behind anyway. So, upgrade now!
Looking forward, the Windows 10 Fall Creators Update introduces the Window Mixed Reality experience to customers, and new features like Timeline and OneDrive Files On-Demand. The major Windows 10 update will also bring with it a host of security enhancements, bug fixes and overall system performance tweaks to improve your user experience.