Hmm, so far I think stuff is pretty well documented.

The listing of subjects exists at http://student.mit.edu/catalog/index.cgi. The “help” section is helpful (unlike some help sections on other websites) and explains pretty much everything.

Short version:

• Wheat = fall; flower = spring.

• The grey wheat/flower button adds something to your selections / tentative schedule. But clicking one isn’t a commitment or anything; the help even says

  You can use this collection page to select the subjects you find interesting, and then print the page … to keep a permanent reference.

• Units X-Y-Z (e.g. 3-2-7) indicate about X hours for recitation/lecture, Y hours for labs/etc., Z hours for preparing (studying by yourself?) per week. X + Y + Z is how many units of credit the course is worth.
• Prereq(uisites) are courses you must have taken before taking this course.
• Coreq(uisites) are courses you must take concurrently with this course. (I’m guessing it’s also allowed that you’ve taken them earlier because some corequisites would be really weird otherwise for people with advanced standing but I haven’t gotten anybody to tell me yet.)
• A solid Forbidden sign (slashed circle) means it’s not available this year. A fuzzy Forbidden sign means it won’t be available next year so maybe get it while it’s available.