Max is an application and visual programming language for music and video. It is produced by Cycling '74 in San Francisco and is also known as Max/MSP and Max/MSP/Jitter. It can be purchased both standalone and is also bundled with the "Suite" edition of Ableton Live.
It essentially functions as a visual "patch panel", allowing you to add modules and connect them in various combinations and configurations.
Here's a short video introduction that barely scratches the surface of Max's capabilities:
Here are some books that dive much deeper into the capabilities of Max:
The Max/MSP website is at https://cycling74.com/products/max