It's a card that provides lounge access at 1000's of lounges around the world, some airline-affiliated and some independent. Most/many of us have it as a benefit of a credit card (mine with Chase Sapphire Reserve). The benefit ranges from the lowest level of membership with a fee for every club access, or fee-free access for a fixed number of visits per year, or unlimited access (Sapphire Reserve, which also provides access to the small, but nice, collection of Sapphire Lounges). Generally, membership covers 2 people, but the Sapphire-linked PP provides for 3.

Just search for "Priority Pass" for membership details (after checking to see if your credit card includes membership...mostly with higher annual fee credit cards). There is also a Lounge Finder option which allows you to see what lounges are available with membership.