[-] collapses the replies so that you can easily see what's next. On reddit the equivalent functionality is a button literally called "hide child comments."

But as another comment mentioned, you can click root | parent | prev | next to get navigated through the thread. Personally, I prefer the collapse, though, as having the top of my screen move without my direct control is a bit jarring, but there's more functionality from these buttons.

Doesn’t [-] set it to zero?

https://en.m.wikipedia.org/wiki/Brainfuck

Important fact: The [-] does not remove a point. After you get 500 points, you earn the privilege of down voting, you will get a "down triangle" underneath the "up vote up triangle" that you can click on.