a person who spends their days next to the computer all day long. Moles usually spend their time cooped up in their little holes and so this relates to how the "moleman" people are on the computer for the whole day.
you can also use this expression for a woman.. molewoman
Bob: Hey Sharyll what did you do last weekend
Sharyll: Nothing really just stayed in my room and browsed the internet.
Bob: what a molewoman.