What is Marshmallow Fluff?

I don't think I've ever knowingly had marshmallow fluff before. I've seen its iconic red-top packaging many times before, but the idea of a gooey, already-melted marshmallow spread just never really appealed to me (I do like normal marshmallows though).

I had no idea that the invention of this food had such a storied history with Boston and other parts of Massachusetts serving as the homebase backdrop of this sweet treat. The history of this food is so deep that there's an entire book written about it called Fluff: The Sticky Sweet Story of an American Icon.