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.

The other answers basically state that yes, there are bogus conferences (and journals) as well as legit conferences but with very low quality standards. One thing they don't really address, and that I think motivates the OP's question is: why would there be such conferences?

The answer is quite simple. They're a great business opportunity (in a very cynical sense) to capitalize on the "publish or perish" principle of the academic world.

Academics need to publish their work, and early grad students are particularly in need of getting publications on their CV. When the work is low quality, respectable venues won't publish it. So that's where these more or less shady venues come in. These welcoming venues offer the opportunity to publish papers with very limited review, but for sizable fees. Then, for papers to be accepted in conference proceedings one author must attend and present, for another sizable fee. Research grants pay for it, so you get free travel... many of the conferences advertize great locations (e.g. Recife, Brazil, was in a recent CFP I saw).

These publications are then on your CV, and will withstand some basic scrutiny, in the sense that it takes some research to know what conferences or journals are "predatory", "paper dumps", etc. There are a zillion conferences and journals, you can't know all of them... People outside of academia may not even be aware that such things exist (OP for example)...

So, to be clear, if you want to have a respectable name for yourself in academia, you don't want to publish there.... but if that's not your immediate concern, then these venues offer a valuable service to some people. And a great disservice to the scientific community, by channelling funds the wrong way, driving down quality standards, cluttering up cyberspace with useless publications, etc.

I smell a big fat scam:

• The number of scientists from reputable universities that are supposedly attending this is just a little bit suspicious.
• The website is horrible.
• The conference does not exist on IEEE's website when I search for a list of conferences in Spain.
• I'm no engineer, but the paper titles seem nonsensical.

If you're ever unsure, you could find the academic webpage of the professor(s) who are chairing the conference and write an email to the address listed there (making sure that the prof you're contacting is a real person on the website of a real university). They'll either assure you the conference is legit, or be glad for the warning that their name has been harvested and used for the scam.