It is a set of XSLT stylesheets for publishing XHTML 2.0 documents, converting it to the XHTML 1.0 format which can be read by Web browsers.
It is geared towards technical documentation that is mostly static (as opposed to a blog site, or photo gallery, for example), and requires highly-structured, careful markup (as opposed to a Wiki).
The author makes some criticisms of using DocBook for writing technical documentation, and argues for using XHTML 2.0 instead, in this discussion of the documentation format in RDFcat
Of course, it is usually easy to criticize something and much harder to come up with the correct fix. The present program, xhtml2to1, is the author’s attempt at that fix. He hopes that the quality of its output, and the quality of its construction — it is a literate program — will convince the reader that he has succeeded.