Stream sniping is when someone watches your stream to get an upper hand to essentially cheat against you in a game. For example, they could find your position on an open-world map, find out where your base is, or even find out when you're low on health, so they can ambush you. It sucks. The deveopers of Rust, Facepunch Studios... are (to my knowledge), the first game developers to build a preventative measure against it into their game. They call it Streamer Mode.
Hey, its Edward Longface again! Ahhhh!!! Why did he shoot at me?! He probably knew my plan. He must be stream sniping!