"Commander Lilith and the Fight For Sanctuary" is a DLC for Borderlands 2 (and Borderlands: The Handsome Collection) that tells the story of the events that lead up to Borderlands 3. It also adds extra levels (max level cap is now level 80), a new tier of gear and some other awesome stuff, so go get it!
The DLC is available for free for both PC, XB1, PS4, PS4 and Xbox 360, but you will need to claim the game to your account prior to July 8th to get it for free. You will not have to download it, simply head to the digital store of your choice (Xbox Store, Playstation Store, Steam etc.) and find the DLC there. Add it to your cart and check out. Once you have done this, the DLC will permanently be tied to your account, so you can download it and play it in Borderlands 2 at any point in the future, even when it begins to cost money.
We don't know exactly what it will cost after the free time period is over, but it will apparently get a price tag. A few dollars seem likely.