The truss rod is typically made of steel or a similar strong material. It is attached to the neck at the headstock, and the other end is typically anchored to the body of the guitar. The truss rod is turned using a special tool, and this causes it to either tighten or loosen. When the truss rod is tightened, it pulls the neck forward, which causes the strings to rise higher above the frets. When the truss rod is loosened, it allows the neck to relax and the strings to drop lower.
The correct amount of relief in a guitar neck is a matter of personal preference. However, most players agree that the strings should be about 0.01 inches (0.25 mm) above the frets at the 7th fret. If the strings are too high, they will be difficult to play, and they may also buzz. If the strings are too low, they will be too close to the frets, and they may cause fret buzz.
Adjusting the truss rod is a simple task, but it is important to do it carefully. Overtightening the truss rod can damage the neck of the guitar, and it is always best to err on the side of caution. If you are not comfortable adjusting the truss rod yourself, you can take your guitar to a qualified guitar technician.