Each heading that you use generates also a "named anchor" into the HTML file.  This allows you to refer to a section of a wiki page.

For example, if the heading on this page is called
{{{
!!This is my heading
}}}
The resulting HTML code would look like
{{{
<h3><a name="section-NamedHeadings-ThisIsMyHeading">This is my heading</a></h3>
}}}

This allows you to refer to the heading from a different wiki page.  And you can use it from WikiMarkup as well!  For example, saying [TextFormattingRules#PreformattedText] (which looks like this in WikiMarkup: {{{[{TextFormattingRules#PreformattedText]}}}) refers to the "Preformatted text" -section of the page [TextFormattingRules].  If you are too lazy to figure out what the link name is, you can also just type the heading as-is:  [Text formatting rules#Preformatted text].

Note that this does not work with [CamelCase] links.