If you’ve ever installed a PC game from the mid-2000s to early 2010s—think Bioshock , Mass Effect 2 , Fallout: New Vegas , or The Witcher 2 —you’ve probably seen it pop up without a second thought: a small gray window titled “Microsoft DirectX End-User Runtimes (June 2010).”

It does not change the core version of DirectX built into your operating system; it simply adds missing optional libraries used by specific games. Why You Still Need It Today

Microsoft DirectX is a collection of application programming interfaces (APIs) for handling tasks related to multimedia, especially game programming and video, on Microsoft platforms. Historically, DirectX updates were delivered via the "DirectX End-User Runtime" web installer or stand-alone packages.