What Does Layer Mean?
A layer, in the context of object-oriented design, is a set of classes that share the same module dependencies with other modules, implying that layer components are reusable as long as conditions stay the same.
Techopedia Explains Layer
Layers act as a sort of architectural pattern. Tree-form hierarchy is one of the common arrangements of layers, where limbs/links between layers are known as dependency relationships.
Multiple forms of dependency relationships between layers include inheritance, composition and aggregation. There are other forms of dependencies in other architectures.