The diagram to the left depicts the Onion Architecture.  The main premise is that it controls coupling.  The fundamental rule is that all code can depend on layers more central, but code cannot depend on layers further out from the core.  In other words, all coupling is toward the center.   This architecture is unashamedly biased toward object-oriented programming, and it puts objects before all others.
In the very center we see the Domain Model, which represents the state and behavior combination that models truth for the organization.  Around the Domain Model are other layers with more behavior.  The number of layers in the application core will vary, but remember that the Domain Model is the very center, and since all coupling is toward the center, the Domain Model is only coupled to itself.  The first layer around the Domain Model is typically where we would find interfaces that provide object saving and retrieving behavior, called repository interfaces.  The object saving behavior is not in the application core, however, because it typically involves a database.  Only the interface is in the application core.  Out on the edges we see UI, Infrastructure, and Tests.  The outer layer is reserved for things that change often.  These things should be intentionally isolated from the application core.  Out on the edge, we would find a class that implements a repository interface.  This class is coupled to a particular method of data access, and that is why it resides outside the application core.  This class implements the repository interface and is thereby coupled to it.
The Onion Architecture relies heavily on the Dependency Inversion principle.  The application core needs implementation of core interfaces, and if those implementing classes reside at the edges of the application, we need some mechanism for injecting that code at runtime so the application can do something useful.