There's the "Courier's Stash" and "Gun Runners Arsenal" DLCs. Which one are you referring to?
As Farmerk said, the "Courier's Stash" dumps items into your inventory at the start of the game. As soon as you wake up on Doc Mitchell's table you start getting notifications of items being added to your inventory. That DLC gives you items that were previously only available by preordering the game before release. There were 4 different preorder packs available. Which one you got depended on which retailer you preordered from. With this DLC you can get the items from all 4 preorder packs at once. When I got this, I only wanted items from two of the packs so I unchecked the other two in my load order and those items loaded anyway. I had to remove them from my data folder to keep the excess items from loading.
Also, as Farmerk said, the "Gun Runners Arsenal" adds new weapons, mods, and ammo to merchants all over the wasteland. None of it will be found randomly in the game. To "experience it 100%" you need to have a lot of caps!