Brain Damage

I think it's about being labeled as insane for being different, and then being driven insane by being forced to conform.

.....The lunatic is on the grass .....The lunatic is on the grass .....remembering games and daisy chains and laughs .....got to keep the loonies on the path

He spent his childhood (games), grew up (daisy chains, meaning series of connected experiences), and found happiness (laughs) in his own way (on the grass). But since he is different, he's considered insane and is forced to live by the rest of society's standards (on the path).

.....The lunatic is in the hall .....the lunatics are in the hall .....the paper holds their folded faces to the floor .....and every day the paper boy brings more

Folded faces = the people on the front page of the newspaper. He is examined and ostracized for being different.

.....The lunatic is in my head .....The lunatic is in my head .....you raise the blade, you make the change .....you rearrange me 'till I'm sane

The lunatic is in my head = His way of thinking is considered crazy. Doctors treat him (or tool with him—"raise the blade, make the change" as in, for example, adjusting a table saw to do wood work) until he is thinking the way society wants him to.

.....you lock the door .....and throw away the key .....there's someone in my head but it's not me

They have succeeded in changing him. He knows the person he has become is not his true self, but they force this upon him anyway, literally driving him out of his mind.

I think the meaning of "dark side of the moon" is found in Eclipse. Everything under the sun is in tune (everything you do goes together to make up who you are), but the sun is eclipsed by the moon (you can be completely separated from who you are). When the sun is eclipsed by the moon, the Earth is on the dark side of the moon.

So when he says:

.....And if the dam breaks open many years too soon .....and if there is no room upon the hill .....and if your head explodes with dark forbodings too .....I'll see you on the dark side of the moon

and:

.....And if the cloud bursts, thunder in your ear .....you shout and no one seems to hear .....and if the band you're in starts playing different tunes .....I'll see you on the dark side of the moon

I think he's basically saying "when you can't take it anymore and finally lose it, I'll see you there; they got me too."