Watch the Visualizing Major Scales & Modes online guitar lesson by Allen Hinds from Melodic Improv
You reach a point where having some theory knowledge really comes in handy. You may be the next great blues guitar player who only needs to know the pentatonic scale in one place, and that's all OK. But, the genius of guys like Robben Ford is that they take that blues roots approach and apply modal and harmonic knowledge found in jazz.
One thing that most of my students have problems with on the fretboard is the modes. They don't know if they need to learn all of them, how they're used, why they need to learn them, etc. Guitar is a strange instrument since it's tuned in fourths, so it can be hard to visualize every major scale on the fretboard. So, a great way to start is by looking at one string and drawing a template. You can really see the whole-whole-half-whole, etc. pattern that the major scale is made up of when looking at it this way. You'll also start to see the sharps/flats that appear in the major scale when moving it around when learning it this way.
But, what happens when you start on the second degree of the scale? If we follow our template, you will see that this is the start of the second mode. The modes are just a fancy way of saying, "start on a different degree of the major scale". In the next lesson, we'll talk about why this is useful.