All guitar necks must have a slight concave bow in them. This gives the strings room to vibrite without "fretting out'' on the upper frets. However if the neck is bowed too much the action will be too high and the guitar difficult to play.
By tightening the Truss Rod you make it a little shorter which caused the headstock to be pulled back slightly lessoning the depth of the bow. The best thing to do is to touch the low E string down on the highest fret (don't play it, just rest it on the fret) look at the middle of the neck, there should be a slight gap between the string and the fretboard. If the string lays flat on ALL the frets, you need more of a bow. (Loosen the Truss Rod)
To put it simply: Too much of a bow in the neck, TIGHTEN THE ROD
No bow in the neck, LOOSEN THE ROD.
