Access levels allow you to display fields in a form to selected usertypes.
Access Level - Admin only
So, for example, you create a field named Remarks to let administrators add remarks about a certain attendee.
Of course, you would want to keep it private and only show it to other admins with access to the backend platform. So you would edit the field and select Admin only under Access level. That will make sure only Admin level users have access to this field.
Access Level - Logged-in users + Admin
To display a field only to users who're logged-in to the website and the admin users in the backend platform, select 'Logged-in User + Admin' in the Access control for the field you're editing.
Access Level - Public
Set access level to Public to display the fields to users who are not logged in on the website i.e. unregistered, guest users who're visiting your website. That can be used to display public profile and to display fields which need to be filled for registration.