Said as an expression to signify when there is indeed a difference between two things or situations, but that difference is unimportant or negligible. Often used when someone is making a big deal out of an insignificant difference. Can also be used to indicate indifference between two choices.

Origin of the phrase: Generally, if someone wants a cheeseburger, and they're served a hamburger, they won't fuss or complain. Sure, there's a difference, but who cares? It's a slice of cheese. Your life's not going to end because you didn't get cheese. Get over it.

Briefly, "the realization that each random passerby is living a life as vivid and complex as your own." Originally from the Dictionary of Obscure Sorrows, which has a lot more beautiful neologism definitions like this you might enjoy.