This directory contains C language bindings for the Mojo Public API.
The environment/ subdirectory defines some common things that, while not part of the system API, may be required for GLES2 (for example). These are things that a Mojo application may be required to provide to the GLES2 (for example) library in order to use it. (However, the Mojo application may implement these things as it sees fit.)
The gles2/ subdirectory defines the GLES2 C API that's available to Mojo applications. To use GLES2, Mojo applications must link against a dynamic library (the exact mechanism being platform-dependent) and use the header files in this directory as well as the standard Khronos GLES2 header files.
The reason for this, rather than providing GLES2 using the standard Mojo IPC mechanism, is performance: The protocol (and transport mechanisms) used to communicate with the Mojo GLES2 service is not stable nor "public" (mainly for performance reasons), and using the dynamic library shields the application from changes to the underlying system.
The system/ subdirectory provides definitions of the basic low-level API used by all Mojo applications (whether directly or indirectly). These consist primarily of the IPC primitives used to communicate with Mojo services.
Though the message protocol is stable, the implementation of the transport is not, and access to the IPC mechanisms must be via the primitives defined in this directory.
This directory contains a C API for running tests. This API is only available under special, specific test conditions. It is not meant for general use by Mojo applications.