From Halopedia, the Halo wiki
An ambient AI refers to an artificial intelligence system in Halo games that have a programmed animation that confines them to a certain area which they cannot leave. These AIs are not to be confused with the AI characters like Cortana from the games and books.
Ambient AIs are typically used to populate a given playspace in a game to provide the illusion of a living world. A simple example of this would be the overhead battle between the UNSC and the Covenant in the Halo: Reach's campaign level Tip of the Spear. For the most part, they ignore the player unless approached, and can be killed just like any other AI enemies or allies. Instances that include them are the technicians in Crow's Nest, the wounded Marines in Tsavo Highway and The Storm, the Banshees and Hornets in various levels, and the Final Grunt, among others.
Killing an ambient AI counts against the metagame score if the player kills allies, however, killing ambient enemies will add points to the metagame score.