PIF stands for pay it forward, possibly based on the horrible movie with Kevin Spacey, Helen Hunt, and Haley Joel Osment. The idea is to "pay" a good deed forward by offering an item for free. Then whoever gets the item is expected to either offer one of their items for free, or, if they are only a buyer on etsy, to do a good deed for someone else.
Since the minimum cost for an esty item is 20 cents, the item is listed for 20 cents, but the buyer is not normally expected to pay the fee. Sometimes, the buyer is asked to pay for shipping, but not normally.
Pay it Forward originated when an etsy seller gifted an item to mellowbeing because another seller had done something nice for her. Mellowbeing in turn offered a pearl necklace for free to whomever was interested. The idea caught on, with etsy buyers grabbing the free PIFs, and then most often offering one of their items for free in turn.
There are at least three places where PIFs are found. First is the original thread, which is still alive more than 3000 PIFs later. There are also often treasuries that list PIFs. Finally, you can search the items: tag, titles for "PIF" and quite a few will show up.
Now days, most PIFs are simple items, samples, or things that someone wants to get rid of anyway. But lots of people love getting and giving PIFs. There's even quite a bit of controversy, while people try to defend the original intent of PIFs--proposing rules about how to give and get the gifts.