It’s an expression of shock and excitement. It comes from a specific image that became a Twitch emote of the same name. If you look up “pog” or “pogchamp”, you should find the image. The face pretty much speaks for itself. It’s one of the staples of Twitch chat culture
The term has also evolved in everyday speech as essentially being equivalent to “sick” or “dope”
Just say you want Trump to win, we can all tell.