QuickTime is Apple’s media player software. It was originally developed for Mac only, but eventually Apple produced a Windows version. It’s often installed on Windows systems as it’s almost the only way to play Apple’s proprietary Quicktime media.
The current version of Quicktime for Windows has at least two security vulnerabilities. Rather than fix those issues, Apple has decided to stop developing the Windows version. In other words, if Quicktime is installed on your computer, it is – and will always be – vulnerable.