Fields allow content administrators to enter data into the backend of their GovCMS site. The fields available vary depending on the content type, however some fields are common (and mandatory) across all content types, including a Title.
Some fields are optional, for example the body field in a Webform.
Field settings, such as whether a field is optional or mandatory and the associated widget type are pre-configured by your site builder when building the site. For example, for an Event content type in GovCMS, the start and end date and time are logical and necessary, so they are configured to be mandatory fields.
A property is set at a deeper level (sometimes referred to as "baked in"). For example, all content has a date it was created on, and this date property plus any associated functionality is fixed. It is part of GovCMS and the underlying Drupal CMS.
The screenshot below shows both the Author property and the Last saved property which are visible to the author when editing the Event.