QuickTime was a program by Apple that was used to run video files. Apple has discontinued support for the program which means they are no longer making updates for the program including not making updates to fix security holes.
Windows since 2009 and all the newer versions of Internet browsers (Internet Explorer, Chrome, Firefox, Edge, etc) have built in them their own plug-ins to allow you to view video files. This means you no longer need QuickTime on your PC
Because of the wide distribution of QuickTime (essentially everyone had it) it makes a program that hackers like to use to hack in computers. There are known security holes in it and Apple is not patching those holes.
Since it is no longer needed and is also a security risk for your computer, computer experts are suggesting everyone remove it.
To remove QuickTime go to Programs and features in Control Panel (or programs and uninstall programs in Control Panel depending on way you view objects in Control Panel.). Find QuickTime on the list, right click on it and choose uninstall