No, I don’t know. If I had to guess, si might be short for something like “source identifier” and just contain some unique identifier to track who originally shared a link that others are using.

It encodes the contents of the search bar.

E.g. for https://www.youtube.com/watch?v=jNQXAC9IVRw&pp=ygUTZmlyc3QgeW91dHViZSB2aWRlbw%3D%3D

ygUTZmlyc3QgeW91dHViZSB2aWRlbw== is first youtube video in base64, which is what I typed into the search bar to find that video.

Not sure if that’s actually useful for tracking or if there’s another technical reason for it, but at the very least it could accidentally expose your search terms to others if you end up sharing links like that.