A Truss Rod (trə́s rɑ́d) is a rod that runs under the fret board of a guitar that is used to stabilize the neck of the instrument so that string tension is constant. Truss rods can be made of wood or graphite, but are most frequently made from steel. There is no standard location for a truss on a guitar. On older guitars, the truss tends to be hidden; on newer models, the truss is usually just above the headstock or slightly inset by the point where the tuners are located. 

The curvature of a guitar’s neck is influenced by a variety of factors including humidity, seasonal changes and even the gauge of the strings that are being used.  When loosened, the truss rod is used to slightly bend the neck of the guitar and, when tightened, the truss rod straightens the neck.