It’s an API, a set of function declarations and tools, which gives a standard way to control GPUs to make them put stuff on the screen as well as 3D maths. DirectX is a proprietary standard owned and developed by Microsoft and as such only supported on Windows. Though we have libraries such as dxvk which act as DirectX drivers and instead of directly interacting with the GPU, use Vulkan, which is similar to DirectX but and open standard, to put stuff on the screen.