I really love his work. I couldn't tell you what they are about either. It's been a while since I watched Inland Empire and Blue Velvent, but besides the plot, the surrealism is more about feelings or visual poetry or something. It feels subversive and critical of the liberal facade of society.
I think if it was easy to put into words what they are about, they wouldn't be art. Lynch creates things that cannot easily be defined by writing them down. Like the saying "A picture is worth a thousand words".