From Halopedia, the Halo wiki
Forerunner sentinels that can be called in by Anders to cause area denial.
Though their primary purpose is to maintain, repair and protect the Ark, these Forerunner Sentinels can be summoned by professor Anders, who has found a way to deploy them in the service of the UNSC. Once called, the Sentinels swarm into a targeted location and attack any hostiles in the area with lasers beams, remaining and causing area denial for a duration before returning to their jobs on the Ark. Any Sentinels that are destroyed emit an EMP blast, crippling hostile vehicles temporarily.
The Sentinels are worker drones with basic AI, created by the Forerunners to safeguard their stations as well as perform repairs and maintenance. There is a large variety of Sentinels in both purpose, size and appearance. There are small Constructor drones used to maintain Forerunner structures, huge Retrievers used for mining and Aggressor drones for attacking any perceived threat. The Sentinels will only attack if they perceive a present threat; otherwise they will happily ignore strangers and carry on with their work.