'Stream sniping' is the practice in which a watching player attempts to join, and possibly disrupt, a streamer's game.
What's strange about this episode is that Tfue seems to have been stream sniped by his own teammates, after trying out the 'fill' feature in sqauds.
It all took place during Tfue's February 26 stream. After joining the match with three random players, Tenney could hear his own stream through one of his teammates' headsets. At this point he called out his teammate for stream sniping, but they quickly denied the allegations.
One of the players accused of stream sniping had the username 'FaZe FarkasSM', and admitted that, of course, he was not actually a member of the organization - but that didn't stop Tfue from joking, "You play for FaZe? I've heard those guys are trash."
Tfue heard his stream again during the same game of Fortnite, confirming his suspicions - although it is possible that his teammates simply turned his stream on after discovering they had luckily managed to land in the same lobby.
As one of the most popular Fortnite streamers on the platform, Tfue has to put up with stream snipers regularly - although typically they are enemy players causing a nuisance, which given his in-game skill, the FaZe pro can deal with.
In future though, Tfue might think twice about trying the 'fill' feature for finding matches, since he's bound to have at least a few stream sniping attempts, trying to match in the same game.