With the table of contents you can present a simple linked list of the headings on a certain web page. The portlet collects all templates that are marked as H1-H6 and creates a list of these.
To create a table of contents you have to use the templates that are set up to be used for primary heading level H1-H6 in order for them to be visible in the list. To be sure on what templates to use, ask the systems administrator.