Regarding "discussion sections" for undergraduate science courses
I can say with 100% confidence that it will depend on the professor(s).  That being said, you should expect to:


Reteach all content.  You won't actually have to, but you should of course be able to.  Students often use discussion sections to ask questions they don't want to ask the prof for fear of "looking dumb."
Give meaningful practice problems and explanations the students should familiarize themselves with for an exam.
For some classes, actually lead a discussion of topics.
Just use it for office hours.