It's not rocket science is considered to be one of the worst clichés in the English language (there is even a book about it - see Amazon.com).
The phrase is used to emphasise that something is not that difficult.
Rocket science is now part of a wider field of aerospace engineering, with principles based on physics and in particular, fluid dynamics.
It's actually not that difficult.
Neuro-surgery on the other hand...
EDIT - Thanks to batarista for this clip from the BBC's That Mitchell and Webb Look