Aimee,
Your injury sounds like a disc is involved and I believe it's the L5S1 level the doctor was talking about, esp. because of your symptoms.
A great website that explains so very much is
www.chirogeek.com
go to herniated discs/disc bulge, or whatever you should choose to investigate so you can get some detailed explanations to your questions. I don't know if you will find it too overwhelming to read from this website as one friend I referred told me it was "over his head" but there is a wealth of information there.
Very simply put~ between your vertebrae there are these cushions so to speak, discs that are gelatinous in material and when there has been a severe insult to the fibrous material around the discs the material can leak out and irritate the involved nerves at that level (L5S1 in your case according to your doc or to be revealed with MRI).
The material that is noxious to the nerves creates the symptoms that you feel, which could be burning, numbness and so forth.
Now that is a super simplified explanation and what you will read at chirogeeks website is the educational answers to your questions (not to mention the correct answers).
Once you've familiarized yourself with the spine and the components and your symptoms, dermatomes affected and such, you can go back to your doctor on your appt. day and ask all the questions but have an idea of the answers you might receive altho I must admit that most of the information I've ever gotten about my back in 20 plus years has come from other sources than my doctor's visits as they gave much more simplified answers than I even replied above to my questions!
Also, don't get fixated on any one thing until you have your MRI which is a magnetic resonance imagining ~ just google MRI and read about that and I think chirogeek has some info on that too. Actually he does have an MRI page that you can check out for explanation. Check under Disc Bulge for info on discs~
Good luck, rest up and don't do anything right now to upset the disc further, just try to relax and allow for muscles spasms to stop, ice the area intermittantly and/or use heat if that helps you more.
I'm very sorry you had this injury occur however I do hope that because you are young and otherwise healthy that however it's treated, you'll recover well. Lifting boxes and heavy stuff is probably out tho so you must be careful there.