Short answers, pulled from the story.
Midgard means "middle enclosure" or "middle yard," derived from the Common Germanic compound Meðjana-garðaz. In Norse mythology it refers both to Earth as the realm of humans and to the wall the gods built from the eyebrows of the jötunn Ymir to defend the world against the jötnar.
The gods slew Ymir, the first created being, and placed his body into the central void of the universe. His flesh became the land, his blood the oceans, his bones the mountains, his teeth the cliffs, his hairs the trees, and his brains the clouds. Four dwarfs named Nordri, Sudri, Austri, and Vestri held up Ymir's skull as the dome of heaven.
The Midgard Serpent is another name for Jörmungandr, also called the World Serpent. According to the Eddas, Jörmungandr will arise from the ocean at Ragnarök, poisoning land and sea with its venom and causing the seas to surge against the shores before the final battle on the plain of Vígríðr.
At Ragnarök, Midgard and almost all life on it will be destroyed, with the earth sinking into the sea. The Eddas also describe the earth rising again afterward, fertile and green, when the cycle of creation begins anew.
J. R. R. Tolkien, an Old English scholar, was inspired by references to middangeard and Éarendel in the Old English poem Crist A. The Old English middangeard is the same word as Old Norse Miðgarðr, and its Middle English form middellærd or Mittelerde became "Middle-earth," the name Tolkien used for the setting of The Lord of the Rings.
Sö 56 is a runestone inscription from Fyrby that uses the word Miðgarðr in a commemorative poem. It names Hásteinn and Holmstein as brothers and calls them the most rune-skilled men in Middle Earth, recording that they placed the stone in memory of their father Freysteinn.