A permission allows you to define the kind of rights a group of users will have to the documents within a series or app.
For an instance, if you have an app called "Invoices" and you want you colleagues from Accounting to work on those files, you need to add a new permission for them. At least they will need Read rights in order to be able to see those invoices.
To add a new permission, go to the Permissions tab. Click on "New Permission".
Then complete the form and click on Create.
Remember: At least users will need Read permissions to be able to see the documents that belong to a series.