It’s very difficult to hit every pitch you’re singing spot-on. You may be extremely close, but you’re probably not hitting the EXACT note. There is often some flatness or sharpness to a singer’s pitch. Autotune is a pitch correction software that can help the notes be closer to the exact pitch the singer is trying to hit. If you use it too heavily, you get the robot-style sound as an effect. If you use it lightly, it will nudge your voice towards the pitch center of the note you’re trying to hit.