If you just want to install it, then head over to the GitHub project page for download links and instructions.
In Ableton Live, once you assign an automation source to a control, you cannot assign additional automation sources for that control. There are situations where it would be nice to have two sources for one parameter, for instance an LFO at a vibrato frequency modulating filter cutoff, while larger manual filter cutoff moves are provided by a MIDI control surface.
When combining signals, you have to choose how to combine them. Do you add them, subtract, multiply, divide? There are situations where each would be useful.
This device allows you to combine two automation sources using a mathematical operator, and assign that output to control another device parameter. That output can be scaled, shifted, or inverted.
Have a look at this video for a demo:
Detailed instructions, version history, and download links are at the GitHub project page. Head on over there to continue.