Microsoft refactored their core app runtimes in 2015. They have made what they call a “Universal C Runtime” which has been distributed via Windows Update to all supported versions of Windows that stay up-to-date (which is the default behavior for Windows Update).
Starting with Xojo 2016r1, the Xojo Windows framework has been updated to use the latest Microsoft tools. This allows Xojo to stay up to date and allows the Windows support to be improved in future releases. This means that Xojo itself now uses the new Universal Runtime and your built apps now require it.
Most users will not notice a change. As long as Windows Update has been keeping their systems up-to-date, then they will already have the Windows Runtime installed.
But as we all know, not all users keep things up-to-date. For users that do not have the Windows Runtime installed already, they will need to install it before they can use your apps.
The easiest way to manage this is to have your app installer include the Windows Runtime redistributable installer and run it automatically. If this is not practical, you can also supply the specific DLLs as part of your installer.
For more information on this, refer to the Windows Universal Runtime topic in the Dev Center: http://developer.xojo.com/windows-universal-runtime
To learn how to create an Inno Setup script for your Xojo app that will automatically include the Windows Universal Runtime redistributable (and other things), watch the Windows Installer webinar: http://developer.xojo.com/webinar-windows-installers