Unicorn is an HTTP server for applications that deal with well-performing clients on connections that show low latency and have enough bandwidth. It takes advantage of features that are present in the core of Linux-like systems. It is called a Rack HTTP server because it implements HTTP for Rack applications. Rack, in turn, is actually a Ruby implementation of a minimal interface to deal with web requests, which you can use in your code.

Unicorn runs as a daemon server in Unix and is programmed in Ruby and the C programming language. Using Ruby means that it can also run a Ruby on Rails application such as GitLab. 

Now that you have a grasp of the basic concepts of Unicorn, we can install it, and also connect NGINX, which we installed earlier, to it.