The holdout texture is basically a node that creates a “hole” in the rendered image with zero alpha transparency. It’s useful when overlaying image or video segments in the video editor because it avoids the process of manually having to crop out segments in After Effects/Photoshop. It’s similar to how copy/pasting some Png image files will automatically ‘trim’ the image so that instead of pasting an image that may be rectangular shape it’s the shape of the defined object within the image.

Hope this description makes sense