Welcome to Brainstuff from how stuff marks dot com, where smart happens. Marshmallows are a confection a candy. They've been around in the form we know them today since the mid eighteen hundreds. They're called marshmallows because the early recipe called for sap from the root of a marshmallow plant.
According to Merriam Webster's Collegiate Dictionary, a marshmallow is either one a pink flowered European perennial herb of the mallow family that is naturalized in the Eastern United States and has a musilanginus roots, sometimes used in confectionery and in medicine, or to a confection made from the root of the marshmallow, or from corn syrup, sugar, albumen, and gelatin beaten into a light, spongy consistency. That word mucilaginous means jelly like.
Later the roots step was replaced by gelatine, and that's how modern marshmallows are made today. For more on this and thousands of other topics because it how stuff works dot Com