Lots of art says something about what we understand, some of it says something about how we understand. Useful art maybe says a bit of both. And also: We don't think that's elitist, or arrogant, or deliberately difficult. It's just realistic: and (we think) a fact of receivership – objects reach out and grab us, and we try to understand them. I for one am regularly faced with the implications of our own sense of lack: my lack of knowledge, my lack of understanding, my potential for total and complete misunderstanding. What is this lack? Do objects (films, performances, whatever) contain everything that is needed to understand them? Or do We have to bring something, some supplement to them? So but then, we can ask: what is an art of lack? Something that proposes no clear or proper way in which it can be understood?
 Something that requires us to put in a shift, w/r/t comprehension?
 Something that resists understanding?
 Something that (by coming into contact with other ways of thinking – like maybe philosophy, performance…) suggests that art can be something other than art? 
Something that pushes thought to it's delirious maximum?
 Abstraction that comes full circle back to realism? Who knows. Here's a nice quote: (Hollis Frampton to Carle Andre):
“I here accuse you of believing that art, first and foremost, should be elevating. I suggest instead that we elevate ourselves.”