"DirectX 12 is a ground-breaking Microsoft technology that gives game developers the ability to harness the full capabilities of a PC’s GPU(s) and raise the bar on graphic fidelity and immersive gameplay," Microsoft explains in the video's description.
The neat thing about DX12 is that it's a low level API, like Vulkan. What that means is that developers have deeper access to CPU and GPU resources, which in turn reduces overhead and offers better programming efficiency. The end result is better performance—Microsoft claims a 20 percent bump in GPU performance and up to a 50 percent improvement in overall CPU usage in its promotional video.
It's tough to show the benefits of DX12 in a video, though Microsoft tries with various game footage. At different points in the video, DX11 footage is transformed to DX12 visuals. The changes in contrast and color saturation are a bit deceiving, and some of the bumps in quality shown in the video are attainable with DX11 as well.
Nevertheless, DirectX 12 has merits, even if they're difficult to portray in a promotional video. It's something we'll see over time as more games take advantage of the API.