Is there such thing as "paper dump conference"? If yes, what actually is it?
I think there are two different but related kinds of conferences. 

First are the conferences which are essentially bogus and shady. They do exist and run but the associations and quality they claim is fake. These are borderline fraud conferences but hard to prove so because they will come up with some bare minimum genuine-ness criteria if called out. People I know usually avoid them and their CFPs go to spam mail.

Second are the conferences which are genuine but fairly low on popularity and quality. Such conferences are often easy targets if you want to get something out in some catalog and want a speaking experience. As an anecdote, my professors used to call these second type of conferences the "paper dump" venues.