Rack is a modular interface between web servers and web applications developed in the Ruby programming language. With Rack, application programming interfaces (APIs) for web frameworks and middleware are wrapped into a single method call handling HTTP requests and responses.

Rack is used by many Ruby web frameworks and libraries, such as Ruby on Rails and Sinatra. It is available as a Ruby Gem. Many Ruby applications are called "rack-compliant".

Rack has inspired similar frameworks in JavaScript (jack.js), Clojure, Perl (Plack), Common Lisp (Clack), and .NET (OWIN).

The characteristics of a Rack application is that the application object responds to the call method. The call method takes in the environment object as argument and returns the Rack response object.

Source:

The environment that is taken as argument by the call method refers to an object that has:
a) Information on the HTTP Request

This includes the information like:

b) Rack specific information