This directory contains “public” interfaces and libraries for various Mojo services (whose implementations are elsewhere, including in particular //services; some implementations may be in separate repos).
(Note that “core”, i.e. absolutely essential, interfaces are under //mojo/public/interfaces.)
The majority of the contents under this directory are mojom files describing the interfaces, located under <service_name>/interfaces. Note that some subdirectories do not correspond directly to a “service” per se, but to a more general group of interfaces (which may in turn be used by other “services”).
There are also some language-specific libraries (especially for the “client” side, but occasionally also for the implementation side) in corresponding subdirectories. For example, C++ libraries are under <service_name>/cpp.