From Tolkien Gateway

The Slag-hills were great mounds of debris, composed of blasted stone and earth, that had been piled over many years by toiling Orcs in front of the Morannon.[1]

On 4 March T.A. 3019, Frodo, Sam, and Gollum came to the Slag-hills and hid in a hollow.[2] The next day Frodo prepared to enter Mordor by the Black Gate but was persuaded by Gollum to head south to seek a more secret way through the mountains.[3]

Approaching the Black Gate on 25 March 3019, Aragorn arrayed his meager forces on two of the Slag-hills behind a "moat" of reeking mud and foul-smelling pools. It was upon these low hills that the Army of the West was surrounded and attacked by the forces of Sauron in the Battle of the Morannon, and it was here that they witnessed the downfall of Sauron when Frodo completed his quest.[1]