Check
here for a Wikipedia entry that explains it nicely.
Slashdotted refers to getting your story posted on slashdot. It's a favored haven of geeks throughout the world for interesting news and stories... many tech-related, but some not.
If your site is posted, what then happens is the millions of slashdot readers check out the sites related to those five articles. The resulting attack of web page requests has been known to crash all but the most robust of servers...
... and when that happens, it's referred to as being slashdotted.