The properties of a document can be exported as metadata on an HTML page easily. This metadata such as keywords or DC.description is used by web crawlers or search engines to enhance user experience or the search rank ratings.
To enable the rendering as metadata, add some Velocity statements to the 'Main Layout'.
As admin user, select 'Layout' in the 'General configuration'
Then click 'Create custom' (if you have not already applied changes to the layout) or 'Edit' (if there are already changes to the default provided by Confluence)
If you have not customized your layout yet, you need to click on "Create custom" to create one instead of "Edit" as shown above.
Finally add the above snippet after the rendering of the meta elements
Save the changes
Note that you need to define metadata sets to actually map properties to metadata keys used in the HTML header. Without this mapping properties get exported with keys that match their name (e.g. Tags instead of keywords).
Add HTML meta tags to your Confluence pages. This is for the search engine optimization (SEO) for your website as well as adding semantics for machine reading.