Xslthl is an implementation of syntax highlighting as an extension module for Java based XSLT processors. So if you have, for example, an article about programming written in DocBook, code examples can be automatically syntax highlighted when you use DocBook XSL to transform the DocBook code to HTML, Rtf, etc.
DocBook XSL ships with built in support for syntax highlighting using xslthl. But xslthl is not limited to be used just with DocBook Xsl, you can use it for anything that used XSLT and one of the supported XSLT processors. More information about how to use xslthl with your own xslt processing can be found on the wiki, just like information about how to configure xslthl to support new languages.