I just started reading the first part of Card College.
I have a question, what exactly is theory in magic?
What does it mean when something is a theory?
Is it sort of a "reason" or purpose for the move?
I'm a bit confused on what it truly means.
The whole idea of magic theory is something that can get confusing. If you expect magic theory to be like a scientific theory, you might be disappointed because many aspects of magic (from the performance to the spectator reactions) are very subjective. Anyway, I'm not going to get into what makes a theory scientific as that is a whole other ball game.
When I think of magic theory, I usually think of the following things
1) It should provide a model that enables you to abstract the problem(s) you face as a performer.
2) It should provide you with broad concepts that are applicable in a variety of situations and are aimed at answering some fundamental questions. E.g., Questions about the purpose of magic and how to achieve it.
3) It should be about more than just moves/mechanics. From what I have seen, it is more about understanding the rules of the game i.e., the way things are set up between you the performer and the audience than about the mechanics/specific details of sleights.
For example, a lot of stuff written about misdirection would fall under the category of magic theory. Also, essays that talk about the essential aspects of magic, like Paul Harris' essay on astonishment as a natural state of mind is also in this category.
Edit: You can read Paul Harris' essay here and see if it makes sense to you.
http://www.bunnyboycollective.com/the-magic-way/astonishment.html
Sometimes people wonder if thinking about such things has any value at all within magic. I guess it is upto the individual to decide.