AI stands for Artificial Inteligence. These are usually NPCs in Halo like Grunts or other non-player controlled characters. AIs can be added to maps via Halo Custom Edition.
What is an AI mod?Edit
An AI mod is a version of a map with added AI in it. Using the Halo Editing Kit, along with HEK+, you can add AI to any ma you desire, and also modify them, giving characters custom colours, weapons, or even completely new characters, which need a Biped model first.