Set the neck to be nearly straight. Tightening the truss will bring the frets in the middle of the neck higher in relation to the 1st and 22nd fret. Loosening the truss rod will lower the frets near the middle. The idea is to have the middle frets nearly as high as the end frets. You can check by holding the string down at the 1st and last fret, then check the clearance at the middle of the neck.
Slight clearance is good.
No clearance is to tight. DO NOT tighten the truss more than a half turn at a time.