What is Ruby? What is the use?
It is a programming language.
In the world of web applications, it is most often combined with a Ruby framework called "Rails" that provides a structure and common functionality useful for building server-side web applications. In this context, it is referred to as "Ruby on Rails", but the "Ruby" part is the programming language itself; so "Ruby on Rails" is essentially the same thing as saying "PHP on CodeIgniter" or "Java on Struts".