Our HOWTOs are released to the public in various formats: Plain Text, HTML, PostScript, and PDF. Instead of having to write the same HOWTO in all of these formats, just one HOWTO is written in a source format, DocBook or LinuxDoc, which gets converted by computer into all of the other formats.
To get an idea of what a source format looks like, take a look at the source file of a webpage (if you haven't already). You will see all sorts of words in <angle brackets>. These are called tags. These webpages (tags and all) are in html: Hypertext Markup Language. The LDP uses formats something like this for its documents.
The markup languages LDP uses meet the requirements of either Standard Generalized Markup Language (SGML) or XML. The LDP now uses the following two flavors of sgml: LinuxDoc and DocBook as well as the DocBook flavor of XML. Interestingly, it turns out that html is just another flavor of sgml (but some features people use in html violate sgml rules so it's not pure sgml anymore).
This mini-HOWTO is all about using the simple LinuxDoc flavor of sgml. You may call it "LinuxDoc markup". It can be converted by computer to html, plain text, postscript, pdf, and DocBook. It's a lot easier than HTML or DocBook and you don't need a special editor for it as it's easy to type in the tags (or use macros for them) using your favorite editor or word processor.