This walkthrough clears up a distinction that trips up a lot of Ableton users: automation sets a parameter to an absolute value, while modulation scales that value relatively to wherever it's currently set. Both live inside the Envelopes box in Clip View, accessed by double-clicking a clip in Session View.
The demo uses a track's panorama control to make the difference concrete. Drawing a modulation curve pushes the pan right, then left, then back to center, but the actual behavior depends on the underlying absolute pan setting, since modulation moves relative to that base value rather than overriding it.
Move the absolute pan position and the same modulation curve produces a different result in the stereo field. Understanding this relationship matters any time you're layering movement onto a parameter that might also change elsewhere, since modulation and automation interact rather than working in isolation.