MPE stands for MIDI Polyphonic Expression and describes a standard for encoding multi-dimensional values into a MIDI note/event. Typically with a normal MIDI controller, you have values like key, volume etc on an event level, and then channel wide values like pitch bend.

With MPE, you can think of it as each note having its own channel, with added dimensions like pitch bend, vibrato, timbre and more all encoded into a note.

This allows you to interface with new kinds of controllers like the Roli Seaboard for example, which are much more expressive than your standard midi keyboard.

In terms of Live, MPE wasn't supported until now and connecting an MPE controller to the DAW was a massive headache and super hacky.