Stock photography generally refers to images that are licensed for specific purposes or to fill specific needs. For example an advertiser might need a photo of a beach to put on a billboard. Instead of hiring a photographer to travel to a beach and shoot the sand it's often cheaper to just license a pre-existing photograph. There are literally millions of stock photos these days and you can buy them or license them online.
Also, there are a bunch of great places to get free stock photography online. One of the most useful is the websites stock.xchng.
Another useful resource is to use Creative Commons photographs from Flickr. Just be sure to honor the license and respect the attribution of the image.
A final thing to do is check out PhotoShelter's archive.
Other photography pages of interest: