The permissions to access or edit pages of your site are defined at group level, that means, here you can set very detailled access rights, and afterwards you assign the user accounts to the different groups.
The group edit view can be switched between default and advanced view. Please notice that all not-saved changes get lost when switching from default to advanced and vice-versa.
Here you can select the group you want to edit:
Enter a meaningful name for the group. This group name will appear at the page settings as a selectable option for frontend access and backend edit of the page.
Here you define which parts of the website are accessable by the members of the group. Depending on the granted permissions, f.ex. "Admin Tools" are (not) displayed to the members of a group.
In the advanced view of the system permissions you can define the access rights very detailled. That means, you can create a group which may edit, but not create pages, has only reading access to the media directory and so on.
"Level 0" at pages means: if you grant this right, users of this group can create pages on the top level of your site; otherwise, they can only create childpages of existing pages.
For groups with the permission to create new pages you can define which modules may be used by the members of this group. This has no impact on whether group members can see existing sections of these modules or not.
You can define which templates may be used by a group which has permissions to create new pages.
This has no impact on whether group members can see pages with certain templates or not.