Hi - I'm not afraid, although I do feel for you as I struggled to understand all the books we studied for English Lit AS level! I went into the exams not knowing what they were really about!
As you've still got a little time, is your English teacher someone who you can sit down with and ask any questions you have about the book? Or how about friends/others in your class who are retaking? Either ask someone to explain it to you, or gather a few people around and have a revision session.
There are a couple of sites who try and give an overview of the book, though they don't go into alot of detail:
http://www.gradesaver.com/classicnotes/titles/ canterbury/section15.html
http://www.bookwolf.com/Free_Booknotes/Canterb ury_Tales_by_Geoffrey_C/Franklin_s_Tale-Canter bury_Tal/franklin_s_tale-canterbury_tal.html
Do you have one of these study guides? Or anything similar to help?
http://www.amazon.co.uk/Franklins-Geoffrey-Cha ucer-Notes-Advanced/dp/0582414695/sr=1-1/qid=1 168095492/ref=sr_1_1/026-8092476-2242844?ie=UT F8&s=books
Good luck!