This is the reverse of ADHD. I most people, their brain produces almost enough internal stimulation to keep itself happy. They still need a small amount to get there though. This can be external (books, films, conversation etc) or internal. In ADHD, the level is low enough that it can't be pulled up enough internally, leading to the short term novelty seeking that is the classic symptom.
At the other end, is this. For some people, the level of natural stimulation is enough to be happy, independent of external (or even internal) sources. They don't need the boost. They are happy sitting staring at a wall, and possibly see it as a nice break from the overstimulation of normal life.
I can do it, I wish I knew how to be better at it. It's almost like a form of meditation. If you let your mind wonder, sometimes it won't stop and will create a little movie or book like experience in your mind. This can last up to 15 minutes for me where I can sit in a trance, completely absorbed in whatever thing is happening in my head. I have seen people who can do it for hours, possibly indefinitely. I deeply envy them.
I have ADHD and my husband has... whatever this is. He stares at the wall sometimes, just quietly and contentedly thinking on the couch and it's really unnerving. I could never