Trussrod

From GuitarWiki

A rod which runs through the neck of the guitar and is tightened or loosened to adjust the curvature of the neck. A hex nut denotes the end of the trussrod where an allen wrench can tighten or loosen the trussrod.

Caution: rotating the trussrod can ruin your guitar! It should only be done in small rotations and only by those who know what they are doing!

If you examine the Fretboard between the 7th and 12th Frets, you should be able to tell if it bows outward, or if it curves inward. Tightening the trussrod will help if this area curves in, and loosening it will help if this area bows out.