DirectX is a selection of application programming interfaces provided by Microsoft and integrates numerous multimedia and namely graphical functions/effects (3D, smoke, lights, etc.) especially related to video games. The letter "X" is a replacement for all the other APIs that are used to start with "Direct" (for example, Direct3D, DirectDraw, or DirectMusic).

Disclaimer: the versions after DirectX 11 are included in Windows 8/Server 2016/10, and there are no stand-alone packages. Every DirectX update is available through Windows Update.

What are the key features of DirectX 12?

Gaming: As mentioned above, DirectX is especially related to video games. It is perfect for powering PC and Xbox games, and mostly 4K games on Xbox One X.
Debugging: With this latest version, developers have access to the contents of resource memory - this is great for spotting errors in the code and the time doing so is optimized.
The CPU-GPU Divide: DirectX can communicate with the drivers of famous GPU manufacturers (AMD, Nvidia, and Intel) across devices. We can say that DirectX is the layer between the CPU and GPU drivers.
Commands: Figuring out which command crashed during a game takes less time - DirectX records the executed commands, as well as those that are in the process of execution and this information is still available after the crash occurs.