With current technology, folding displays can either fold inwards (i.e. mating the screen surfaces) or outwards (mating the back surfaces). Either operation effectively doubles the screen real estate.

Samsung Galaxy Z Fold3

Huawei Mate Xs

No manufacturer can yet make a screen that folds both inwards and outwards along the same hinge. But Samsung has figured out to make a screen that can fold both ways along separate hinges. This means that they can triple the screen size, as compared to the object in its folded-up form:

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Right now it's just a proof-of-concept and not part of a production model; it's being displayed by Samsung at the 2021 International Meeting on Information Display conference in Korea. But "The day we carry smartphones like a [powder compact] or wallet," the company writes, "is coming soon!"