Slashtags are a way to build specific search domains. The easiest way to understand one is to build one for yourself.

Example: I built myself a /objc slashtag that includes Apple's docs, Stackoverflow, Cocoadev.com, Mike Ash's blog, and a few other sources.

Now, when I go to blekko and search, say, "calendars /objc", I get only info about calendars in objective-c from those sources, and no noise.

Edit: I find this rather more useful than DDGs ! Syntax in that it's user-extensible, and lets me aggregate many site searches into one slashtag, rather than just punting me to a specific site's engine.