What military item is the roman corbus?

It is spelt corvus. It was a bridge on the prow of Roman ships during the First Punic War. It could be raised an lowered with pulleys. It was used to board enemy ships. The Romans adopted it to make up for their naval inexperience (they were fighting their first naval war) against the great skills of the Carthaginian sailors. Instead of making a complicated manoeuvre to ram the enemy ship, they sailed by its side and boarded it. It worked. However, the device was useless in rough seas and compromised the ship's manoeuvrability. It is likely that it made the sips vulnerable in a storm and that the sinking of dozens of ships in storms and the loss thousands of Roman lives was due to the corvus. it was eventually abandoned.
It is not to be confused with the corvus belli, figures for wargames and collectors.