@lucifargundam @Acer

I think they're referring to dead links (link rot).

The internet archive is used to store web contents, so if the contents were archived before the web site at that link went down (or before the contents were otherwise removed for the link), then a link to the archived site should still have it. But if the link was archived after the contents "rotted" then it would have the contents at that point (or an error message).