From Halopedia, the Halo wiki
A camping stool is a small, collapsible stool used by the humans to provide relatively comfortable seating in field conditions.
These stools are light and easily portable, and collapse so many of them can be stored in a small space. The seat itself is strung between the frame, and appears to be made out of some green material.
These stools can be found on Halo 3 and Halo: Reach's campaign and multiplayer levels. They are often found near sniper rifles and camp spots. They are also an editable object in Forge found under the "Scenery" tab and can be added to a map at the cost of $2.