The object preview allows you to configure a preview URL for your objects, you can do that in the basic configuration of your class (see screen below).
This URL can be a normal detail-page of your object, for example reachable via a custom route.
You can define placeholders in your preview URL, for example the object ID or some custom properties of the class. You can use every property defined in the class even the properties which are added automatically by pimcore, for example o_id, o_key, ...
Placeholders have the same syntax as you already know from the custom routes, just put a % in front of the property name.
When opening the preview tab in an object, the placeholders are replaced with the current values of the object and the URL is opened in the tab.
Only if you configure a preview URL in the class configuration, the preview tab is shown!
Example
The above configuration (/news/%name%o_id{_}) will result in the URL /news/my+news+title_867655 (or whatever the object id is).