Eternal Draft Packs are card packs, which rather than containing cards from a specific expansion set, contain select cards from expansions prior to the most recent one. The card pool is curated by DWD to both support the current Draft format, and also define the legal cards for the constructed Expedition format.
Like normal expansion packs, Draft Packs each contain 8 Common cards, 3 Uncommons, and one Rare or Legendary card.