Why is everyone on the Internet saying 'F'? 

Press F to pay respects is essentially a sarcastic way to say 'R.I.P.' on the Internet. It originated from the 2014 game Call of Duty: Advanced Warfare in a scene where you are asked to hit the 'F' key to pay respects to a fallen soldier (or 'X' if you are playing on a console). 

Shortly after, people on the Internet started using it on platforms like Twitch when a player died in a game. People also use it if someone gets verbally destroyed on social media or something similar. 

Check out Know Your Meme for more information on "Press 'F' to Pay Respects."