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). 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)
Until all shared mailboxes are migrated at the end of February 2022, a shared mailbox can be either a Microsoft 365 account or a Connect (aka on-prem) account.