We'll have to start with the remarkable video. Wikipedia's description is below.
It was shot near Liverpool Street Station in London, and intersperses footage of Radiohead playing the song inside an apartment with scenes of a middle-aged man who lies down in the middle of the pavement just outside of the apartment building. People start to gather, thinking that something must be wrong with the man, and the band are shown looking out the window at the events below. A heated (subtitled) conversation between the man and the crowd develops, as the people start demanding to know what the man is doing and why he is lying there. In subtitles, the man finally gives in and says, "Yes I'll tell you, I'll tell you why I'm lying here... but God forgive me... and God help us all... because you don't know what you ask of me." The camera zooms in on his mouth as the man finally gives the answer, but the subtitles have suddenly stopped, so the reason is not revealed to the viewer. As the camera zooms back out, it shows the pavement covered with the crowd of people, all lying down just like the man.
The official video is here, another similar version below.
I'm curious to learn the great horrible secret shared by the middle-aged man, but I doubt I'll be able to; I've learned that everyone involved in the creation of the video was executed for humanity's sake.
The song itself is great. It is multi-layered and complex, dense with guitars--the fantastic screeching guitar solo by Jonny Greenwood has to have been sampled at some point--and anchored by Thom Yorke's odd tenor, fairly high and with a near-whine that's so muhc better than anything Johnny Rotten (say) coiuld have done. And then, there are the lyrics.
Can't get the stink off
He's been hanging round for days
Comes like a comet
Suckered you but not your friends
One day he'll get to you
And teach you how to be a holy cow
[. . .]
Don't get my sympathy
Hanging out the 15th floor
changed the locks three times
He still comes reeling through the door
one day i'll get to you
And teach you how to get to purest hell
Yorke has apparently gone on the record as saying that this song is about a narcissistic friend who kept driving people away with his self-regard. The verses support this; so does the refrain, repeated at the end of the song.
You do it to yourself, you do
and that's what really hurts
You do it to yourself, just you
you and no-one else
You do it to yourself
You do it to yourself
Elsewhere, listeners have point out--at the lyrics site above, and in a discussion at songmeanings.net--that this could relate to others sorts of issues, to a depression that forces its way in, or people who try to postpone or avoid recognition of their sins which keep coming back regardless.
The one thing that bothers me about the song is that there doesn't seem to be any possibility of the subject's redemption. That might not fit the song's theme, or I might be missing something, but this bothers me. Could the protagonist stop doing it to himself? Is that what the refrain's about? I wonder.