wow you didn't search very well... oh well I'll explain it. The truss rod is an adjustable metal rod running thru the middle of the guitar neck. It's purpose is to counteract the tension of the strings, which are pulling the neck up and trying to bend it. You can adjust the truss rod to slightly change the bend in the guitar neck, tightening it straightens the neck out and loosening it makes it resist the strings less and bends the neck more. It was invented so guitar necks could be made smaller and easier to play without them bending way out of shape. (invted in the 1920s i think by gibson.)