It is a compilation of the four item packs that were previously released as bonuses for pre-ordering the game at certain retailers. It costs $1.99 (£1.23), or 160 Microsoft Points. It contains multiple new weapons, armors, and a supply of aid items, such as Stimpaks and Doctor's Bags.
Any file that you have will automatically add it to your pack when loaded, if you have already downloaded it.
Downloading this while playing any of the add-ons will still add the items to your inventory.
The items given to the player are relatively good for early levels, but are surpassed by better weapons later in the game. Even so, players should not underestimate these items, as they can give a great push early on in the game.