From his recordings, it's a form of improvisation. It's like new age music that doesn't really go anywhere; more atmospheric than idea driven. Most classical musicians probably wouldn't like it but non-musicians seem to dig it.
It sounds very unsophisticated to my ears as it could go on for hours, it seems. And if you've heard one piece, you've really heard them all. It's nothing extraordinary as far as improvisation goes. Keith Jarrett does a much better job.