It means "working a desk job". These types of jobs usually involve sitting at a desk, working on the computer or answering phones all day. It seems to be used in kind of an insulting way, such as a police officer saying "Tony joined the police force to catch criminals, but all he has done is ride a desk. I guess they don't trust him with a gun".
EDIT: It is not very common, and many native speakers would probably have to tell by context what it means.