The Most Important Step In Landing A Plane: The Cross Check

"Flight attendants prepare doors and cross check"

What the hell is a cross check? What are the consequences of not doing a cross check? If the pilot forgets to signal to the flight attendants to do the cross check, does everyone just stand around and hang out?

I'm guessing that it's a glance out the window to make sure that the gate is actually there. If so, kinda sad that they have to be reminded. "Aw man, remember that time I opened the door and let people off and there was no gate there? Everyone just fell off the plane. It sucked."