True Stereo panning in Ableton Live lets you move a sound's actual position between the left and right speakers, instead of just shifting its volume balance like default panning does. Access it by right-clicking the pan dial on any track in the Mixer.
With a drum loop and synth on the same track, you can hear the difference directly: panning the drums leaves them locked in the center, while sweeping the synth's dial sends it drifting across the stereo field independently. This makes it possible to place multiple elements at distinct points in the stereo image without them all moving together.