It means that your website is mentioned on a extremely popular website called slashdot.org. Slashdot brings an avalanche on visitors, which can crash servers, fry CPUs, damage hard drives, etc... Well, not really, but I am sure that happened :)
Depending on your niche market, SD may not be a very good source of traffic. Slashdotters are usually geeky/techy people that are up to date with technology. In some research they said that they come and leave, they don't stay long on a website, and certanly don't click on ads.