This template just adds a heading above the SubPageList parser function call.
By transcluding this template with {{Subpages}}, you get
== Subpages ==
{{#subpages:}}
wherever you want to show a list of links to subpages.
The parser hook used comes from the mw:Extension:SubPageList extension, and the documentation gives further examples of customizing the output https://github.com/ProfessionalWiki/SubPageList/blob/master/doc/USAGE.md
