From Halopedia, the Halo wiki
A destroyer is a type of warship in both the UNSC and Covenant navies. In seaborne navies, a destroyer has been defined as a fast and maneuverable yet long-endurance warship intended to escort larger vessels in a fleet or battle group and defend them against smaller, short-range but powerful attackers (such as a fighters).
The destroyers used by the UNSC are roughly the same size as a UNSC frigate, but are designed with greater emphasis on firepower and armor over versatility. Aside from using them as fleet escorts, the UNSC is known to deploy destroyers on patrol and scouting operations.
The Covenant navy also includes vessels classified as destroyers. Covenant destroyers are employed in a variety of roles, including serving as escorts for larger ships such as carriers, whilst being themselves escorted by smaller ships like frigates. The Covenant also use destroyers in planetary glassing operations.
The Forerunners possessed an analogous ship classification, although their definition of a destroyer is significantly different from that used by the younger races. For example, the experimental flagship UNSC Infinity and the CAS-class assault carrier Shadow of Intent, both highly formidable command vessels, are considered mere destroyers by the Forerunner definition.