These obelisks are made of sandstone and Haradric Brick. They consist of three trenches ringed with walls of increasing height, surrounding a central column. Each obelisk is topped with a burning hearth block. These structures are purely decorative, and don't contain anything valuable for players to find.
Harad obelisks were likely built by the ancient Southrons in the early days of their civilization. As such, they were likely intended as monuments to the Dark Lord Sauron.