CRTC is an antiquated term that stands for cathode ray tube controller, which today would be simply named display controller as CRT monitors have disappeared and many other display types are available. The CRTC is an abstraction that is responsible for composing the frame to be scanned out to the display and setting the mode of the display.
In the Xilinx DRM driver, the CRTC is represented by the video mixer. The bottom-most plane is the primary plane (or master layer) and configured statically in the device-tree. The primary plane always matches the currently configured display resolution set by the CRTC (width and height) with X- and Y-offsets set to 0. The primary plane can be overlayed with up to eight overlay planes inside the video mixer.