People already defined it as meaning there is little or no difference between things being compared. I can't comment, so I am answering with my input.
A good example (meaning about the same thing) is "six of one, half-dozen of the other." (A dozen being 12)
Another is "To-MAY-to, to-MAH-to".
All of these are informal, but imply that they are the same thing. It is sort of a way to dismiss a straw-man argument, or more likely someone nitpicking details in a story.
Me: "My cousin Joe is a dumb hillbilly. He crashed his truck into a fencepost while drinking Bud Light!"
Insufferable bore: "Actually, he's a redneck; hillbillies come from the Appalachians. And he was drinking Keystone Ice."
Me: "Same difference."
This exchange implies that the details don't matter; his cousin is a 'low-class' drunk, regardless of the particulars.