A basic understanding of XML is needed because meta-metadata is an XML based language. If needed please see an XML tutorial.
The ability to form XPath expressions is necessary. These expressions are heavily used to retrieve data from HTML pages. XPath tutorial.
Regular expression are used often in meta-metadata for specifying either URLs or parts of text. Regex tutorial.
Application Developers Only - Java
The ability to understand and program in Java, C#, and/or Objective C is crucial for creating applications using meta-metadata. The meta-metadata compiler runs in Java, and generates classes in Java, C#, and Objective C.