The first thing you need to know is that exposure meters are "stupid", really very "stupid". They do not have any idea of what they photograph and they assume that all the scenes have the same tonal value: middle gray (famous gray of 18%). For example without correction, a snowy scene will be underexposed. If you follow scrupulously the indications of the exposure meter, each scene will have the same middle gray density and you will get lots of bad exposures, especially if you photograph in difficult light.
Ansel Adams developed the Zone system to cope with this situation! It provides photographers with a simple and effective method to define with precision the relationship between what they see and the results that they will get. It allows for a direct correlation between the visual world and the real photographic print. Originally the system was developed for film, but is easily applicable to digital photography.
To know : lthe only way to a complete understanding of the zone system is through trial and error. You must try and try before complete mastery of the subject.