The word for the human world in Old Norse is a description of a farm.
Miðgarðr is mið, middle, plus garðr, an enclosure — the fenced-in home field of a Scandinavian farmstead, as opposed to the útgarðr beyond it. What people lived in was not "the earth" or "creation" but the middle enclosure: the bit inside the fence, with everything else outside it.
That single compound carries more of the Norse worldview than any myth does. There is an inside and an outside; the inside is small; and the fence is the whole of civilisation.
Made from eyebrows
Grímnismál stanza 41 says the gods made Miðgarðr from Ymir's brows for the sons of men, and Snorri repeats it: the sons of Borr took the eyelashes or brows of Ymir and made from them the stronghold called Miðgarðr, around the earth, as a defence against the hostility of the giants.
Read literally, that means the fence is a piece of the corpse the world was cut out of. The gods made a wall out of the face of the being they killed, and put humanity inside it. Norse cosmology does not distinguish between building and butchering.
Snorri also says the earth is round and the encircling ocean lies about it, and that the gods gave the giants the shores to live on — which produces the picture that everything about Miðgarðr assumes: a disc of habitable land, a fence, a coast populated by hostile powers, and the deep sea beyond that with Jörmungandr in it.
The first people
Völuspá has three gods find Askr and Embla on the shore, without breath, sense, fate or colour, and give them what they lack — see Hœnir. The names mean ash and, probably, elm, though the second is disputed and has also been read as vine or as a word for a creeper.
Snorri's version has the sons of Borr walking along the sea-strand and finding two trees, which they take up and shape into people. The materials in both are driftwood or standing timber. The Norse creation of humanity is a woodworking operation performed at the tideline.
Middangeard, and where Tolkien got it
The compound is not confined to Norse. Old English has middangeard, used constantly in Beowulf and in religious poetry for the world of human beings, and Gothic has midjungards in Wulfila's fourth-century Bible translation, rendering the Greek oikoumenē, the inhabited world. Old High German has mittilagart; Old Saxon middilgard.
The word is therefore common Germanic, attested in the fourth century, and used by Christian writers without embarrassment for well over a thousand years. In Middle English it becomes middel-erd and middel-erthe by reanalysis, which is how the modern form arose.
Tolkien, a professional philologist working on exactly this material, took the Old English term directly. He cited the line from the poem Crist — éala éarendel engla beorhtast ofer middangeard monnum sended — as a formative moment, and his Middle-earth is that word, restored to its Germanic sense of the mortal lands, with an actual mythology built around it. It is the single most successful transmission of an Old English word into global culture.
The fence in practice
Every major Norse myth is about the boundary being tested. Thor's function is to keep giants on their side of it. The wall of Ásgarðr is built by a giant under a contract. Útgarða-Loki rules the outyards. Jörmungandr is the boundary, alive, and Thor's repeated attempts on it are attempts on the fence itself.
There is nothing metaphorical about the anxiety. Scandinavian and Icelandic farms were small enclosures of managed land in a very large amount of unmanaged country, and an innangarðs/útangarðs distinction structured Icelandic law — where livestock could graze, where a killing was more or less serious, what was owed and to whom. The cosmology is the legal geography of a farm, expanded until it covers everything.
What survives
Miðgarðr has no cult, no offerings and no personification; it is a place-word rather than a power. The modern word "Midgard" is used in fiction, games and Marvel's films as the name for Earth — which is accurate to the sources in a way most of Marvel's Norse vocabulary is not.
For the enclosure above, see Ásgarðr; for what the world was made of, Ymir; for the boundary itself, Jörmungandr; and for the tradition, the Norse overview.
Is Midgard one of the nine worlds?
It is one of the very few that unambiguously is. The number nine is repeatedly given, but no surviving text lists the nine worlds, and the standard modern roster — Ásgarðr, Vanaheimr, Álfheimr, Miðgarðr, Jötunheimr, Svartálfaheimr, Niðavellir, Niflheimr, Múspellsheimr — is assembled by later readers from names scattered across different poems and Snorri, several of which may be alternative names for the same place. Miðgarðr, Jötunheimr and Ásgarðr are the three that appear consistently and that the narratives actually move between.
Does Midgard survive Ragnarök?
Völuspá says the earth sinks into the sea, and then that the earth rises again out of it, green, with fields growing unsown. Whether that is the same earth restored or a new one is a question the poem does not answer, and the phrasing supports both. Two humans, Líf and Lífthrasir, survive by sheltering in Hoddmímis holt and repopulate the world, feeding on the morning dew — so the category of a middle enclosure with people in it continues, whatever happens to the specific ground.