A truss rod is a metal rod that runs along the neck of stringed instruments, such as guitars and basses. Its primary purpose is to provide support and adjust the neck's curvature, ensuring proper string action and playability. By tightening or loosening the truss rod, musicians can correct any bowing or warping of the neck caused by changes in humidity or temperature. This adjustment helps maintain the instrument's overall sound quality and performance, making the truss rod an essential component for maintaining instruments like guitars and basses.