The default permissions are the permissions you want to apply to all future publications.
This allows you to add groups or people with whom you share the more often to avoid having to add those settings to each new publication that you put on your page.
With each new release, you will see them appear automatically in the "I want to share with:" area.
You can of course change these permissions for each publication!
To create and edit your default permissions, you must go into your settings. To access it, click your picture at the top right (1) in the top menu bar and then on "Settings" (2):
Once in your settings in the left menu, click on "Privacy" then "Permissions Defaults".
You can then add your family and groups in the "Share with" and in the "Do not share" (this option is useful when you want to share with a group, but you want to exclude one person from the group ).
Individuals or groups added here will be automatically added to the permissions of your future publications.
Once finished, do not forget to validate!