I am very confused about focal length's. I know that it is the distance from the center of the lens to the point to where it focuses. I guess my question is why is this important to know? Why do I need a certain setup of matching fresnals and doublet/triptet to make my projection clear? Can I use any arrangement of lenses differently spaced to get the same result?

Any help would be greatly appreciated.

I can try to help you here. You need to know where the lense focuses so you can put what your trying to fucus there! If you were superior or posterior to that position with your object then it would not be in focus. The reason you want your triplet focal length to be very close to that of your collamnator fresnal is you want your triplet to be able to be focused on your lcd right where that light cone converges from the fresnel. About your lense question... No you can not use 'any' lense in a different combination to get the same result. Figure it out mathmatically and you will see why. We also use the parts that we use because they are in very good supply and cheap. If your question is can you make it with 'other' lenses, then sure but it will most likely cost you more than if you use the tried methods, and you are probably the first to 'try' it so there is no gaurantee you will achieve the same results. I should also mention that I am really really new around here.