A violin bow has a screw that allows you to adjust the hairs' tension. Like a tuning peg so to speak.
When I just got mine, I didn't pay attention to it. I worked with the tuning pegs because I knew I would need to, but I didn't touch the violin screw because, well, I have no idea why should I.
Tutorials and such don't seem to talk much about it. All I understood from a quick read around is that it is kind of an early improvement to the bow. This suggests me that it really isn't very necessary in the first place.
I think I also heard that a "proper" tension level is when you can pass a finger through the gap, no more and no less.
Exactly what role does adjusting the violin bow tension play?