The Brief: Typing "f" into a chat is a way for people to "pay their respects" to something or someone.

Bluesquiggle
DEEP DIVE

The term originated after a feature in the 2014 release of Call of Duty asked players to press F on their keyboard or X on their controller to “pay respects” to virtual fallen soldiers.

Since then, the term and action have been co-opted as a way for people to playfully “pay their respects” to any number of things in Twitch chats and other comment threads.