A permission allows you to define the rights that a group of users has over a space. For example, if you have an Invoice space, you can define that the Accounting group has write permissions on the Invoice space.
To add a permission, go to the Permissions tab in the Users menu. Click on the "New Permission" button.
Next, you will need to define what space a group will have rights to and what kind of permissions.
A minimum read permission is required for a user to be able to view documents in a space.