Short answers, pulled from the story.
The Old Norse word Hel derives from the reconstructed Proto-Germanic feminine noun haljō meaning concealed place or the underworld. This term ultimately traces back to the Proto-Indo-European root kel- or kol- which means to cover, conceal, or save.
Snorri writes that Hel is located in Niflheim and exists beneath one of the three roots of the world tree Yggdrasil. The Prose Edda places this realm in the ninth of the Nine Worlds where evil men go upon death.
Hermóðr rides for nine nights through valleys so deep and dark until he arrives at the river Gjöll and crosses the bridge with a roof made of shining gold. He then passes the Gates of Hel and enters the high hall of Hel where Baldr sits in a seat of honor.
Hel the being was cast down into Hel by Odin who made her ruler over Nine Worlds. She gives out lodging and items to those sent to her that have died of disease or old age.
A völva states that Hel will play an important role in Ragnarök when a sooty-red cock crows from its halls to signal the beginning of the event. This rooster is one of three cocks that will announce the start of the final battle alongside Fjalar in Jotunheim and Gullinkambi in Valhalla.