Tachanka is a character in a game called Rainbow Six Siege. He has a big machine gun that he can put on a tripod, kind of like a camera stand. This gun shoots really fast and can do a lot of damage to bad guys.
People who play the game like to use Tachanka because he can defend an area really well. If the bad guys try to come in, Tachanka can shoot at them and keep them from getting in.
But sometimes, people make jokes about Tachanka because he's not always the best choice to use in the game. They say things like "Lord Tachanka" to make fun of how powerful he is, even though he's not really a lord.