It refers to the overall mission of Akane Kurashiki and Sigma Klim working with Crash Keys to have Sigma and Phi, two espers with the ability to SHIFT their consciousness back and forth in time, to 2028. In order to do this, Sigma and Phi need to hone their skills and prevent the Radical-6 apocalypse. The Nonary Game: Ambidex Edition is a part of the AB Project.
It began in 2029, after the C-END: 1 route in the Decision Game, and ended in 2074.
In actuality, Delta is one of those also behind the AB Project in a way. Delta and Akane met back in 2029 at a Free the Soul ceremony. Since Delta could Mind Hack, he likely read Akane's thoughts about the AB Project and they agreed to work together, although Akane would need to keep this secret from everyone else.
The AB Project needed an element of danger and Akane allowed Dio to infiltrate the game and murder her in some timelines. Delta also needed the AB Project to succeed in order to create his timeline where no one dies.