The einherjar are those who have died in battle and are brought to Valhalla by valkyries. In Valhalla, the einherjar eat their fill of the nightly-resurrecting beast Sæhrímnir, and are brought their fill of mead (from the udder of the goat Heiðrún) by valkyries.
The lifestyle of the einherjar would have been the envy of living Vikings. All day long, they fight one another, doing countless valorous deeds along the way. But every evening, all their wounds are healed, and they are restored to full health. However, Odin does not keep the einherjar around just so they can enjoy extravagant pleasures. He has a particular purpose in mind for them, and they’ve all been carefully selected to fulfill this purpose.
The einherjar prepare daily for the events of Ragnarök, when they will advance for an immense battle at the field of Vígríðr; the battle which the "ein" (here meaning single-time) refers to. Odin is fated to fight the dreadful wolf Fenrir. The einherjar will fight valiantly by his side, but despite all their fearless efforts, they are doomed to fall with their master and with the rest of the cosmos as well, for that matter. Heimdallr occasionally returned the best of Einherjar to Midgard or Jotunheim with the purpose of killing giants, but they were forbidden to talk with the living.