You could read the Help file ...or... you could simply try using them to see what they do.
You wont break anything by doing either.
They're simply transport (play/reverse) tools that can be helpful in narrowing down to a particular frame... personally I never use them, and use the CTI instead when I need to move back and forth on the timeline. They're a holdover from hardware based video editing systems, when you actually had a physical wheel that you could spin to make your movements... I think there are still USB devices that will interact with them if you prefer that physical form of moving around a clip. But I'd venture to guess that for 90% of us, they hold no use anymore.