To put it simply, stream sniping is when a viewer purposely gains access to a streamer's in-game lobby with the intention to derail or assist, or even aggravate the streamer.

Is stream sniping illegal?

What is stream sniping? ... The platform's TOS clearly mention that any sort of cheating is bannable on Twitch and stream sniping is considered as such. “Any activity, such as cheating, hacking, botting, or tampering, that gives the account an unfair advantage in an online multiplayer game, is prohibited.

How do players stream snipe?

Stream sniping is when players log onto a streamer's Twitch channel, look at what they're playing and then queue in that game and attempt to join the same lobby as that streamer. They will then watch the channel while playing the game in an attempt to locate and attempt to either kill or grief that streamer in-game.