Shared accounts/mailboxes are typically generic accounts that more than one user has access to. For example, an email account for a specific project, task or department. (ie. a Department account where a number of users could access the email). Access to a shared account can be set up either by:
security group (where a user with permission for the account would use their own credentials to access the account)
a linked account (where the user with permission for the account would use the credentials of the generic account)
Note that a message marked as Private will not show in the Desktop app. The message will only show in the Web version of Outlook.