The default value fills in the field when a new listing is created. It is saved with that listing like any other value. It is not a fallback that shows on listings where the field is empty.

What that means in practice

• New listings, added from the frontend or in wp-admin, start with the default already in the field. If nobody changes it, that value is saved.
• Existing listings are not changed when you add or edit a default. Editing and saving an old listing does not fill in the default either.
• For a checkbox field, the default is whether the box starts ticked or unticked.

Set it at Places > Settings > Custom Fields by opening the field and filling in Default value.

Filling in a value on existing listings

To set the same value on listings you already have, export the listings to CSV, fill in the column, and import the file again. See How to Import/Export Data?.

Showing the same content on every listing

If you want the same text or shortcode on every listing, put it in the display layer instead of in a field value. Add a Shortcode tab at Places > Settings > Tabs, or add the content to your details page template. It then shows on old and new listings, and you change it in one place.

Shortcodes in field values only run in textarea and HTML fields. Fields that listing owners can edit should not hold shortcodes or scripts. How to allow scripts in GD Custom Field Values covers the cases where you need it.