Hi there John, a truss rod is inside the neck of the guitar, basically it's a threaded rod that adjusts at either the headstock or at base of the neck, if you could take the fretboard off from the neck, you would be able to see the whole rod. It's function is to adjust the amount of relief in the neck, a neck must be bowed slightly forward, toward the strings, to play properly because of the way a string vibrates when it's plucked, a perfectly straight neck or one with a slight backbow will cause the strings to contact the frets when plucked, usually around the 7th or 8th fret. Hope that answers your question, if not, feel free to ask........