F means, they want to remain notified for further posts in a particular thread or they want to follow the discussion by members on a particular topic. Some members type F or f or follow or following to be kept in the loop for the discussion.
The phrase "Can I get some F in the chat?" is a meme that originated from the video game "Call of Duty: Advanced Warfare," released in 2014. In a specific scene early in the game, players are prompted to pay their respects to a fallen soldier by pressing the "F" key on their keyboard.