Short answers, pulled from the story.
Ymir is a primeval giant born from venomous drops dripping into the icy rivers of Élivágar within Ginnungagap. He lived as the ancestor of all frost giants, producing male and female jötunn from his armpits and a six-headed being from his legs.
The three gods killed Ymir to fashion Earth from his flesh and seas from his blood while mountains formed from his bones. They created trees from his hair, clouds from his brains, and the heavens from his skull held up by four dwarfs at its corners.
Four specific poems including Völuspá Vafþrúðnismál Grímnismál and Hyndluljóð contain references to Ymir as a primeval being. These texts describe creation events such as the age when he dwelt in the yawning gap and how the gods fashioned the world from his various body parts.
Snorri Sturluson wrote the Prose Edda in the 13th century to structure a cohesive creation myth using fragmented sources. He added details like Auðumbla the primeval cow whose milk fed Ymir and who licked Búri out of ice over three days.
Roman historian Tacitus wrote in his 1st century CE ethnography Germania about Germanic peoples singing songs of a primeval god named Tuisto. Historical linguists have linked Tuisto to Ymir on both linguistic and mythographic grounds suggesting a shared heritage between Norse giants and early Germanic cosmogonic figures.