It’s the old-school term for the “Reddit hug of death”. In other words, huge popular site links to tiny unpopular site, and tiny unpopular site is overwhelmed by extreme levels of traffic it never expected to see and is completely unprepared for, server hosting it melts into a puddle of goo and website becomes inaccessible. (Realistically, server hosting it goes to 100% CPU or memory or both and the website just crashes and doesn’t restart or only functions intermittently and extremely slowly)

Server admin, seeing their server turning to a puddle of molten goo, decides to quickly throw emergency barricades in front of it to try to block enough of the traffic that the server can continue to function, often in vain.

Slashdot.org was the precursor to Reddit for old techies. It still kind of is, but it’s a shadow of what it once was.