This course takes a learning-by-doing approach to theory; in other words, it is doing the activities on your guitar that will help you understand the theory. Simply reading through the lessons will be about as useful as reading through a guitar lesson without actually doing the exercises. Every music theory concept is connected to a specific characteristic of musical sounds. A main goal of theory “lessons” is to get you to the point that you can recognize what it feels like and sounds like to play each concept. Once you can do that, you will be able to use the concept to help you do what you want to do as a guitar player. If you do not play the concepts for yourself, you will not develop that usable connection between the concepts and the sounds.
If a lesson features concepts that you are certain that you already understand and can use, feel free to skip that lesson! On the other hand, if you are finding a particular concept to be a bit elusive, mysterious, or challenging, don't be afraid to linger on that activity or lesson for a while, perhaps even making it part of your normal practice routine until you feel you really “get it.”
Although described as a course, this could easily be used as a textbook.