Becoming a great rock guitar soloist doesn't require a whole lot of musical knowledge. Many very good guitarists stick almost exclusively to pentatonic scales, blues scales, and assorted licks to create their solos. For the slightly more adventurous guitarist, however, there are times when a pentatonic or blues scale just doesn't provide the right sound. This is where modes of the major scale, like the dorian mode, come into play.
If you haven't tackled the modes of the major scale on guitar before, you're in for a whole whack of information to deal with. So, let's put that off for a moment, and just learn the dorian mode shape and basic usage before diving into the music theory behind it.