Make sure the file names end in .sgml. Notice that there are other tags, also enclosed in angle brackets. This is the next paragraph. Note that it is separated from the above paragraph by just a blank line. You are now reading the LinuxDoc flavor of sgml as specified in the very first line of this file.
Tags are anything inside angle brackets.
In later examples you'll see that there are other tags that come in pairs like this. Tags only help the sgml2txt program make the conversion. There are more tags to learn. It's ignored when this source file gets converted to other formats. This is the abstract.