The Mojo Public API is a binary stable API to the Mojo system.
It consists of support for a number of programming languages (with a directory for each support language), some "build" tools and build-time requirements, and interface definitions for Mojo services (specified using an IDL).
Note that there are various subdirectories named tests/. These contain tests of the code in the enclosing directory, and are not meant for use by Mojo applications.
The c/, cpp/, js/ subdirectories define the API for C, C++, and JavaScript, respectively.
The basic principle for these directories is that they consist of the source files that one needs at build/deployment/run time (as appropriate for the language), organized in a natural way for the particular language.
The interfaces/ subdirectory contains Mojo IDL (a.k.a. .mojom) descriptions of standard Mojo services.
The platform/ subdirectory contains any build-time requirements (e.g., static libraries) that may be needed to produce a Mojo application for certain platforms, such as a native shared library or as a NaCl binary.
The tools/ subdirectory contains tools that are useful/necessary at build/deployment time. These tools may be needed (as a practical necessity) to use the API in any given language, e.g., to generate bindings from Mojo IDL files.