Today i read the description of WADL from wikipedia
The Web Application Description Language (WADL) is an XML-based file format that provides a machine-readable description of HTTP-based web applications.
Here what is the meaning of "Machine readable description of HTTP based web applications"?
Machine-readable means that it is written in a way that a program can easily parse it into a well-defined structure.
This way tools like wadl2java can generate "client stub", which is source code that allows programmers to interact with the web application.