XBRL International provides An Introduction to XBRL. That is a pretty good explanation of what XBRL is. XBRL International provides this concise statement as to what XBRL is:
In a nutshell, XBRL provides a language in which reporting terms can be authoritatively defined.
Later in the web page in the section What are some of the most important features of XBRL?; these four important features are highlighted:
The most important thing that I see here is people are finally making the connection between "clear definitions" and "testable business rules". As I have pointed out, business rules are key to achieving a meaningful exchange of information.