The composition probably is the most difficult skill to master in photography and longest to learn. The main issue with the process of learning the composition is its subjectivity, everything is open for the interpretation. In many cases, there is no right or wrong, everything is relative.
It is really important to have a structure or a guideline when learning something complex. This is the reason the rule of thirds is the most important concept when learning the composition in photography. It brings the structure the entire process.
If you ask 10 people to define the rule of thirds you probably get 10 different versions. Luckily we have Joshua Cripps who brilliantly described the rule of thirds in 3 minutes. This the best explanation of the rule of thirds I’ve ever seen.