The answers to this and many other exciting questions that will occur to you can be found by reading the Frequently Asked Questions thread in the About This Message Board forum.
It means “Your Mileage May Vary,” as in the comment at the end of car commercials about what mileage you can expect to get on the open highway from the new 2003 Vroommobile D-Lux with computer-controlled frammistats.
It’s shorthand for “you may see things in a different light, but this is how I see them,” as an acknowledgment of the reader’s right to disagree with the position taken by the poster.
One of my favorite self-depracatory SDMB anecdotes:
Amusingly, back when I was a newbie on this board, somebody wrote a post with a great deal of asperity towards another poster, ending his statement of his POV with “YMMV” – and I was convinced that it stood for “You Make Me Vomit!” :o