<p>When I choose my classes on the planner , it asks me to select the discussion time also. So I wonder what is it?</p>
<p>discussion section is most likely a mandatory part of your class where you will meet with a TA/instructor in a smaller classroom setting to further go over lectures, readings, tests, or whatever your instructor chooses.
i say it is most likely mandatory because in some cases, discussion attendance is not required as a part of your grade (although they are mostly helpful so you should probably go anyways).
be sure to choose a time that works for you because while a some professors let you choose whichever discussion time you can attend, others will require you to attend only the one you signed up for.</p>