YaST is the installation and configuration tool for openSUSE and the SUSE Linux Enterprise distributions.
The YaST landing page located at https://yast.opensuse.org/ provides a clear view of the whole project for both users and developers, but with a stronger focus in the latter.
YaST is a complex project with several components and its source code is spread over several repositories at GitHub, with each repository containing the corresponding development documentation. In addition to that specific documentation, this directory contains some documents targeted to developers and related to the project as a whole. All the documentation is referenced from the already mentioned YaST landing page at https://yast.opensuse.org/, which aggregates links to several sources of information (including the different files in this directory) in a way that makes sense and is easy to digest for newcomers.