AVLD is a V4L dummy video device built to simulate an input video device like a webcam or a video capture card. You just have to send the video stream on it (using, for instance, mplayer or ffmpeg), that's all. Then, you can use this device by watching the video on it with your favorite video player. But, one of the most useful interest, is obviously to use it with a VideoConferencing software to show a video over internet. According to the software you are using, you could also be able to capture your screen in realtime. A third interest, and maybe not the last one, could be to use it with an image processing (or other) software which has been designed to use a video device as input.