It's the cognate to Google, by default, keeping every piece of email you ever sent or received through them, even after you delete it -- and including it in the "storage" of that account.
For several years I've subscribed to both the BOB and 650B group mailing lists. Eventually they had to migrate to Google Groups after Yahoo terminated that aspect of their business (I even remember when it was a Usenet-based thing). I handled this through a Gmail account, filtering it all into an "On My Mac" mailbox for reading, and then throwing it all away when I'd gone through the most recent posts -- or so I believed. I noticed that the Archive folder in IMAP for that account were getting alarmingly large, and realized that Google had been keeping all the BOB-like mail messages, notwithstanding my deletion of them -- my non-paid Google storage was just about exhausted. So I sat down to try to delete the stuff, which had grown to about 100,000 emails between the two lists. If I deleted it from the Archive (which is where Google keeps "all mail", and to which I had paid about zero attention in the past), then it just went to Trash, which had to be emptied again to really get rid of it. Normally, deleting a whole raft of email in Apple Mail takes only a moment -- but this seemed to be going on for hours -- and I realized that it was simply crashing Mail. Eventually, I found a "fix" for this online -- Gmail settings that will (apparently, at least from the user/account point of view) erase them following the mail client's lead. But it was sobering to realize that "delete" or "trash" does not mean "erase" to Google; it's just another "label" (which is Google's version of a "folder" -- they don't really have folders, so you can't just delete a folder with all its contents.
Nothing you ever did on the internet ever goes away (despite the difficulty of finding it again when you want to), and it seems that our (or Big Brother's) propensity for keeping every piece of crap has no practical limitation, in terms of data storage.