Limbo is a religious word meaning a place in the afterlife between heaven and hell.
Thom Yorke has described this song as being 'about when you're leaving and when you come back'.
The original title was 'Lost At Sea'.
Yorke admitted to getting writers block after OK Computer, and said 'I felt I'd totally lost control of any element of my life, or any element of anything that I was involved in, and ultimately being incredibly angry it was inexpressible'.
The following lyrics could explain that period:
'Trapdoors that open'
'I spiral down'
'I'm lost at sea'
'Don't bother me'
Thom has said before that all the songs he's writen were actually nothing to do with him, but they were messages beamed to him.
The lyrics 'I got a message I can't read' probably refer to his writers block.