"Meanwhile, back at the ranch..." is a phrase generally interjected into a conversation when it has veered severely off topic and you'd like to get back to the original point.
It can also be used when you're about to go off on a random tangent or any other placed deemed appropriate for comedic effect.
It alludes to a scene change in Bonanza where the scene would cut from the action to show what was going on back at the Ponderosa accompanied by a voiceover of the narrator saying "Meanwhile, back at the ranch..."
Common variations include:
"Meanwhile, back in my pants..." - popularized by my former roommate during his high school years. He claims that "All the jokes in high school were in [his] pants."