"Meanwhile on Sp00n's screen", or any variation thereof, is a running gag that is mentioned when a video cuts to black for whatever reason. It is mainly said by Seamus.
The phrase originated around episode 14 of Seamus' Killing Floor series when he played with Nova and Sp00n. During the game, Sp00n's computer hit a problem and stopped recording so Sp00n remained in the call though he wasn't playing. Throughout the rest of the game, Seamus and Nova would say things like "Meanwhile on Sp00n's screen, black."
The joke crops up most in Seamus' videos, such as his Halloween Lawl video in which he silently dons his Mario hat and mustache before it cuts to a black screen with white text saying "MEANWHILE ON Sp00n's SCREEN:" before going back to Seamus with a mustache monobrow complaining about the size of his hat. The phrase was recently said by Seamus on Head 2 Head.