The truss rod is attached to the neck at one end and to the heel of the neck at the other end. It is turned with a special wrench to tighten or loosen it. When the truss rod is tightened, it pulls the neck up, which increases the curvature of the neck. When the truss rod is loosened, it allows the neck to relax, which decreases the curvature of the neck.
The proper amount of neck curvature is essential for good playability. Too much curvature can make the strings too high off the fretboard, making it difficult to press down on the strings. Too little curvature can cause the strings to buzz against the frets, which can produce an unpleasant sound.
The truss rod should be adjusted periodically to keep the neck in the proper shape. The frequency with which the truss rod needs to be adjusted will vary depending on the climate and the instrument.