The truss rod is a big metal bar that goes through the whole neck on your guitar and is used to control the amount of arch that the neck of your instrument gets. You will want the neck to be essentially straight, with a tiny bit of curve. Lots of things can cause your wood to arc..weather, storage, age.. and the truss rod is used to fix this and keep the neck straight allowing you to play your piece properly. All guitars come with a truss rod and they are easy to adjust if need be. This tutorial will take you through all of the steps you need to adjust your truss rod and keep your guitar straight. The video also covers how to test your neck and see if it needs to be adjusted.