From Halopedia, the Halo wiki

Interdictors were a type of warship employed by the naval forces of the Forerunner Ecumene. Within the Ecumene's naval classification system of orders of battle, interdictors (alongside their auxiliary brethren) rated as mere sixth- or seventh-order hulls.[1]

The full scope of the interdictors' role in the Forerunner fleet is unknown, though their classification fairly far down the order system suggests they were not considered particularly powerful warships. It is unclear whether the Covenant's own Rasus-pattern interdictor (which is constructed utilising Forerunner relic cores) is considered an interdictor due to a basis on Forerunner interdictors.