Internet skills such as writing html, google searching, customizing a web browser, finding the exact download you're looking for, etc. The word is derived from kungfu.
(after helping a friend find in 5 minutes what he'd been searching for for hours:)
"dude, you've got some major webfu going on"
(when not finding what you want)
"Sorry bro. I just don't have very good webfu today."
(spiffing up a friend's web page)
"Let show you what this can look like after a little webfu"