In a nutshell
A database management system is organised in three layers so different people can work without tripping over each other. The external level is the tailored view each user or app sees; the conceptual level is the single complete logical design — all the tables, fields, and rules; the internal level is how that data is actually stored on disk. Mapping the layers to each other means the design can change without breaking views, and storage can change without breaking the design.