They are the chosen of the fallen: warriors who died in battle and were carried by the Valkyries to Valhalla. The Einherjar fight every day, fall, rise again and feast every night – until the last morning of the world.
The name einherjar means something like ‘those who fight alone (for themselves)’ or ‘those belonging to one host’. They are Odin's harvest: the god gathers the bravest of the dead to have an army for Ragnarök.
Daily life in Valhalla follows an eternal rhythm: by day the Einherjar ride out to the field and fight a mighty battle; whoever falls rises again in the evening. Then they return to the hall of the 540 doors, eat of the boar Sæhrímnir, which renews itself each night, and drink the mead of the goat Heidrun. So they harden themselves for the day when they will ride at Odin's side against the powers of chaos.
“Five hundred doors and forty there are, / I ween, in Valhall's walls; / eight hundred fighters through one door fare / when to war with the wolf they go.”Grímnismál 23, translation after Henry Adams Bellows (public domain)

Valhalla · The Valkyries · Odin · Ragnarök · Helheim.
Grímnismál, Vafþrúðnismál (Poetic Edda), English public domain: Henry Adams Bellows (1923). Snorri Sturluson, Gylfaginning. Scholarship: Rudolf Simek, Dictionary of Northern Mythology; Hilda R. E. Davidson, The Road to Hel.