I wanted to know something about what these Numbers are Telling Me?

If I put a URL of a YouTube Video in Any Video Converter it will Download it.

And I do this just for the Spooky Documentaries.

So Any Video Converter Opens a Window and it Tells Me all the Audio Formats that it can Download it as.

And they Say things like
220 - Very Tiny WebCam Audio Only
520 - Very Tiny WebCam Audio Only.

What is the Numbers In front of all of this?