Threading theory help needed

Hi there everyone, I've written a simple forum in PHP for my website (It's an artistic Jean-Michel Jarre website called Planet Oxygene where I experiment with all things tech) but I want to add threading to my messages but I'm not sure of the theory for such a thing. For example, I know I need ID, messageid, forumid and messagerootid (Just example names) but I'm not sure of the theory of how replies to replies etc ... would work in PHP with MySQL? I guess I'm just confused on the whole threading business and I'm just looking for some good explanation of how I can easily add threading to my messageboard (Assuming my messageboard is just an average system which adds the ID as an auto-numerator, the messageid and all subject etc ...). Any help would really be appreciated just on the basics of how threading works so I can work out the code. Thank you for your help on this, a very confusing subject for me :-) Chris