Environment variable could be defined in Project's environment, usually, Environment variable is used to in url's host, query string and headers.
For example: We have three environment for our API: QA, Stage, Product. Each of environment have different domain name:
QA: http://api-qa.sample.com/ Stage: http://api-stg.sample.com/ Product: http://api.sample.com/
Without variable, we need create three request for these domains, it's too tedious.
We could create a environment named QA and add a variable with key:
Duplicate it and do the same thing for Stage and Product.
Now we get three environments, we can use it in one request.
Create a request with url:
｛｛host｝｝/get, select environment and hit Send button.
｛｛host｝｝ will be replaced by the variable's value of selected environment. So if select QA, request's url will be
http://api-qa.sample.com/, and Stage will be