To tilt something means to adjust its angle. For example, you are hanging a picture on a wall. You ask someone, "Does it look straight?" The other person says, "It's almost perfect. Just tilt it to the right a little."
Recently in video-game culture "tilt" has come to be used to express that a poor performance or loss has placed the player in a bad mental state which will make the game more difficult and less fun. Like: "If this teammate complains to me one more time, he will put me 'on tilt'" or "I'm done playing for tonight. That last game tilted me too much."