archive is a site (there are a few such things) into which you enter URLs of other web pages and it stores (”archives”) them off to separate storage. At least before threadreader does/did not actually archive content, they aggregated it, meaning it could go away whereas archive site stores a copy (subject to copyright restrictions of course) ... my recollection is that FR does not like archive site links.
I see.... I think I’ll leave that to my betters. :-D I have my limits.