Think of modern music theory as the road map to understanding music. Notes are not simply conjured into existence by chance- a good composer has a reason, and a plan for every note they write. Here and here are two links to free websites for practicing basic theory. This will take you an explanation of how the cello works, and this video is an excellent visual for understanding the difference between open tuning (like a cello) and equal-tempered tuning (like a piano).

Go forth and know what you are talking about!