WSGI stands for Web Server Gateway Interface, which is a specification that describes how a web server communicates with web applications or frameworks written in the Python programming language. It was created in 2003 as an implementation-neutral interface between web servers and web applications or frameworks to promote common ground for portable web application development.

WSGI has two sides: the server side and the application side. The server side is responsible for invoking a callable object that is usually defined by a route, while the application side contains the structure and the code that is to be executed. WSGI is a Python standard described in detail in PEP 3333.

WSGI is used to forward requests from a web server, such as Apache or NGINX, to a backend Python web application or framework, and then pass the responses back to the web server to reply to the requestor. It is a mediator that communicates between web servers and Python web applications.

WSGI is important for deploying a Django or Flask app, and it plays a vital role when you deploy your application. WSGI servers promote scaling and are capable of serving thousands of requests at a time.

In summary, WSGI is a specification that describes how a web server communicates with web applications or frameworks written in the Python programming language. It is a mediator that communicates between web servers and Python web applications, and it plays a vital role when deploying a Django or Flask app.