From Halopedia, the Halo wiki
The siege bike is an experimental all-terrain, single-traction tread vehicle designed by the Office of Naval Intelligence's REAP-X department to target well-fortified enemy strongholds and barricades.
The siege bike is equipped with a single side-mounted M268 heavy machine gun equipped with spectrum-enhanced visual tracking functionality. The vehicle is built around a heavy-grade, single-traction tread that allows the bike to be useable in all terrains and achieve speeds of up to 145 kilometres per hour (90 mph), and it is known to be highly maneuverable. The bike is only capable of carrying one operator, who rides atop the vehicle.
The siege bike is one of several experimental vehicles developed by the Office of Naval Intelligence's Reverse Engineering and Prototyping–Xenotechnology department, a subdivision of the agency that focuses on the research, analysis, and reapplication of alien technology. The vehicle's impressive weight-responsive maneuverability is a product of reverse-engineered gravitic modules discovered in a Covenant ship debris field.
Still in the experimental phase of production, the siege bike is explicitly designed to soften enemy fortifications and create breaches that can then be exploited by other groundside forces.