It's good news that you had clear margins and clear nodes.
Did you have a CT scan and bone scan before your surgery? These are the usual scans to have to check that there is no cancer elsewhere.
Chemo decreases your chance of recurrence because it kills any stray cells that may have travelled from the original tumor through the lymph or blood and stops them from reproducing to form a tumor elsewhere. It is very effective against microscopic cells that are not detectable on any scans.
91% is a very reassuring figure to be given. I hope this has answered your questions. I found reading the information about Chemo on this site very helpful in getting an understanding of how it works. You will also be able to ask the oncologist questions when you see them. I liked to try and understand the basics as it helped me to understand why certain treatments were needed.
Take care and hope your next visit goes well.
xx Deanne