The Pelennor Fields were the townlands and fields of Minas Tirith, capital of Gondor.
After Minas Ithil had fallen and been renamed Minas Morgul, the Pelennor Fields were walled by the great wall of Rammas Echor, to prevent an invasion.
During the War of the Ring, the Pelennor Fields were the location of the Battle of the Pelennor Fields, when Sauron's Orcs and evil Men overran the Rammas Echor and besieged the city.
It was on these fields that Théoden King was slain, and that most of the great battle was fought. After the War of the Ring, the Rammas Echor was rebuilt by the Dwarves of Aglarond.
The name Pelennor translates to "Fenced Land" in Sindarin.