NGINX is a scalable web server that allows growth of your website
Nginx - Web server and reverse proxy server that runs on Unix-like operating systems.
In other words:
Nginx - simple, fast and reliable server. Nginx is primarily used for static websites and as a proxy server for dynamic sites. The essence of Nginx is that it quickly handles static data (cached text, pages, and images) and promptly passes this data onto the client.
What is it used for?
If a site has a large amount of static data, (data that is rarely changed on the site, for example, image galleries, videos, or text), the use of Nginx can significantly increase the performance of the website for the end user.