DirectX is one of the APIs that can communicate with the video card on PC. DirectX is available only on Windows operating systems. DirectX also is used for developing Xbox games but we'll concentrate only on PC.
We'll learn modern versions of DirectX - DirectX 11.3 and DirectX 12. It's better to have Windows 10, though Windows 8 and 7 are ok too. DirectX 12 is more complex than 11th version so it's easier to first learn 11 and then learn the new things in 12th.
To follow tutorials you'll need Visual Studio Community - it's free. The latest version is 2019.
DirectX consists of several parts. We are interested mostly in Direct3D - it's the API that manages 3d graphics. But we also look at Direct2D, DirectWrite, and XAudio2. Also, we'll heavily use DirectXMath, it allows us to make fast math.