This signifies the video format (Audio Video Interleave), which was standard for PC video in the 2000s.
From a technical standpoint, the format is a nostalgic reminder of the "pre-cloud" internet. Today, we stream everything instantly. In 2008, if you wanted to see a viral moment from a Stickam stream, you had to download a compressed archive, extract it, and hope you had the right codecs installed on your media player.
This indicates the source of the content. Many users recorded live streams to archive "legendary" or controversial moments.