Hi Ataraxia!
MRSA stands for Methicillin Resistant Staphylococcus Aureus. Methicillin an antibiotic of the penicillin family, heavy duty, and no longer works on this particular Staph bacterium, so we say Resistant. Staph Aureus because that is the name of the species of bacteria involved.
There are a bazillion staph species and Aureus is very common. It can also be very nasty.
We all have a zillion "germs" on our bodies all the time, and one or more of the staph family is probably included in that. Where you don't want them is on the inside; that is why doctors need to wash their hands between patients.
MRSA became resistant to methicillin in hospitals, long term care places like nursing homes and veterans facilities. There is a lot of methicillin being used in these places so a lot of chances for this particular strain of staph to have a lot of experience with it. Bacteria reproduce rapidly and go through a lot of generations in a day, even---so little mutations by which even only one bacterium of the bunch in one infection could survive an antibiotic, were passed down to the next generation, and the next, etc, really fast, and that is how we wind up with a resistant bacterium.
You got colonized on your skin, and now you are being advised to use this wash before your surgery so you can kill it off long enough for them to get in there & do their thing. They don't want to let it get from your skin to your insides, where things will get really bad if it does.
For what it is worth, I read somewhere that some of the more persistant bugs can be knocked down on the skin by washing with a mild--repeat mild---chlorine bleach solution.
http://www.medicalnewstoday.com/articles/147954.php
"Treatment of Staphylococcus aureus Colonization in Atopic Dermatitis Decreases Disease Severity."
Jennifer T. Huang, Melissa Abrams, Brook Tlougan, Alfred Rademaker, and Amy S. Paller.
Pediatrics, May 2009; 123: e808 - e814.
doi:10.1542/peds.2008-2217
They gave kids who had atopic dermatitis baths in mild bleach solution. AD patients often have staph aureus in their lesions.