When you leave google to go to a site and then go back to google without using the back button resulting in google sandwiching in the page youre viewing. Usually a form of protection when viewing a site that you don't want to be caught surfing.
Escaping the page is done simply by using back or forward button either will get you to google.
"Dude youre watching porn at work! NUTS! You better Google sandwich it in case the boss comes back."
Eric: Wow youre seriously watching a Chris Crocker Video? Here comes your girlfriend.
Mike: Shit I better google sandwich it quick!
Eric: Good thinking!
Kelly: So wow this guy I'm dating really is a convict. Here's his mugshot.
barbara: Oh no here he comes! Quick Google it nOW! Act natural!
Kelly: Whew that was close. Good thing I had my google sandwich on.
Hot chick: Ew here he comes. He's gonna want to be invited, quick change the page.
Loser: Wow you guys are surfing google. Cool. I better go back to being lame. Bye.
Hot Chicks sidekick: You TOTALLY fooled him with the google sandwich.