It means something generic instead of a specific font like Verdana. Do something like this:
Code:
font-family: verdana, arial, helvetica, sans-serif;
That is what I use on my website. It'll use Verdana. if it doesn't exist then it goes to arial, then helvetica. If none exist, then it chooses one that is sans-serif. sans-serif is a generic font because it doesn't specifically go to one font, but to any font that is sans-serif. You could also go with something else like serif. I'm not sure of the others off the top of my head though.

Hope that helps.