Inbox is the first of the Main Sections. This is a special folder that has an email address, it is visible when you click on the Inbox section. Sending an email with an attachment (or several) to this address will result in having the file(s) in this Inbox. It will not preserve the sender, subject or other email content, but will only display the attachment(s) the email had.
The email address is unique and you can give it to anyone who needs to send you files. The Inbox section, like any other section and folder can be made visible and usable to your selected team members as well. By default only your account administrators can access it.
This is how the inbox works: