People ask this more often than you would guess, usually after reading that our ancestors foraged all manner of wild greenery, or that reindeer graze moss through the winter. The plain truth is quickly told: true mosses are not food in any useful sense, and most of the famous cases of eating moss turn out to concern something that is not a moss at all. What follows is what is really going on.
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Why moss makes such poor food
A moss is built almost entirely of tough structural material and water, with very little of the starch, sugar or protein that makes a plant nourishing. Its cells are walled in cellulose and reinforced compounds that the human gut cannot break down, so most of what you swallow passes straight through, carrying any small food value away with it. There is no starchy root to dig up and no oily seed to crush, and nothing sweet anywhere about it. Many mosses also carry bitter or mildly toxic secondary chemicals that evolved for the express purpose of putting grazers off, which is part of why so few animals trouble to eat them. Chew a mouthful and you gain next to no energy while asking your stomach to deal with something closer to damp string than to salad.
What people actually ate
Comb through the accounts of famine and survival and the "moss" almost always resolves into a lichen. Iceland moss (Cetraria islandica), long boiled into a bitter porridge or stretched into bread across the far north, is a lichen despite the name; so is the rock tripe (Umbilicaria) that kept more than one Arctic expedition barely alive. Both need long soaking and boiling, often with a change of water or a pinch of wood ash, to leach out acids that would otherwise gripe the gut, and even then they count as starvation rations rather than as nourishment. Genuine mosses barely feature at all. Sphagnum has been eaten in extremity, and a few cultures have chewed particular species, yet as a dependable food moss has essentially no track record, and that absence tells its own story.
The reindeer question
Reindeer moss is the plant most people have in mind, and it too is a lichen (Cladonia rangiferina) rather than a moss. Reindeer manage to live on it through winter because they carry gut microbes and enzymes that ferment the lichen's carbohydrates, machinery we simply do not possess; what sustains a reindeer would leave a person with little beyond a full and unhappy stomach. The whole knot of names is worth untangling on its own, which I have tried to do under reindeer moss versus reindeer lichen.
Where moss did real good: dressing wounds
If moss never fed us, one kind did something arguably more valuable. Sphagnum, the bog moss, holds many times its own weight in fluid and is mildly antiseptic, and it was gathered by the sackload to dress wounds in the First World War when cotton ran short. That use is genuine and worth knowing, but it was a dressing rather than a dinner. The very properties that make sphagnum a fine bandage, its acidity and its spongy, cell-rich structure, are the ones that make it a wretched mouthful. Its longer story sits under sphagnum moss for anyone who wants it.
If you are still tempted
Leave aside the lack of nourishment and there are still sound reasons for caution. Moss growing in the wild soaks up whatever surrounds it, from road salt and traffic grime to the residue of lawn treatments and the droppings of every passing animal, and it is rarely growing anywhere you could call clean. Identification is a hazard of its own, since the small green things people lump together as "moss" take in liverworts and the odd genuinely poisonous seedling. None of this is dietary advice, and if you are foraging for real food there are a hundred better and safer wild plants to learn first. Moss is a thing to study closely and to grow for pleasure, not to put on a plate.