What WSGI¹ is not: a server, a python module, a framework, an API or any kind of software. What it is: an interface specification by which server and application communicate. Both server and application interface sides are specified. It does not exist anywhere else other than as words in the PEP 3333. If an application (or framework or toolkit) is written to the WSGI spec then it will run on any server written to that spec.
WSGI applications (meaning WSGI compliant) can be stacked. Those in the middle of the stack are called middleware and must implement both sides of the WSGI interface, application and server. For the application in top of it it will behave as a server and for the application (or server) bellow as an application.
A WSGI server (meaning WSGI compliant) only receives the request from the client, pass it to the application and then send the response provided by the application to the client. It does nothing else. All the gory details must be supplied by the application or middleware.
It is not necessary to learn the WSGI spec to use frameworks or toolkits. To use middleware one must have a minimum understanding of how to stack them with the application or framework unless it is already integrated in the framework or the framework provides some kind of wrapper to integrate those that are not.
All the code in this tutorial is low level and has the sole purpose to be didactic by showing the WSGI specification at work. It is not meant for real use. For production code use toolkits, frameworks and middleware.
¹ Web Server Gateway Interface