Что такое переменная окружения в ASP.NET ядро?

I studied a part of the asp.net core to build a web application while I am studding, I went to a part that is talked about environment variable,
I want to know what are the environment variable 
When to use then development, staging and production environment variable

I read about them in Microsoft documentation but I didn’t find when to use them and the difference between the three-environment variable