Stream sniping means watching a player's stream with the intention of queueing for a match at the same time as them in order to be paired with them in a match.

In the case of the video linked in the question, the sniper is trying to join the streamer's team in order to "throw", or lose the game on purpose, with the objective of causing distress to the streamer.

Over time the meaning has changed to include gaining an advantage against a player by watching their stream, but the correct term for that is "ghosting", not stream sniping.