There are a few different things that PF can stand for in fantasy, but the most common is probably player character. In RPGs and other similar games, the player character is the main protagonist that the player controls. They're often custom-made to some extent by the player, and they grow and change as the story progresses.
In some cases, particularly in tabletop games, PF can also stand for party funds, meaning the money that all of the players' characters have pooled together. This is generally used to buy items or pay for services within the game.
Finally, it's also worth noting that some people use PF as shorthand for power fantasy. This usually refers to stories or games where the protagonists are incredibly powerful and/or capable, often to an unrealistic degree. These sorts of stories can be enjoyable escapism for many people, but they're not everyone's cup of tea.