YouTube's clip tool lets anyone snip a segment from a video, usually up to 60 seconds, and share it as its own link. That clip link is exactly what this page converts: just the segment's audio, as an MP3.
On an eligible video, viewers click Clip (under the video, next to Share), choose a start and end point, add a title, and publish. YouTube generates a standalone link like youtube.com/clip/xxxx. Anyone with the link can watch that segment.
More cases: why some videos cannot convert.
Other link types: Shorts, live replays, embeds.