These are special user accounts (similar to Microsoft Exchange Room Mailboxes or Shared Mailboxes) that are used for auto attendants and call queues:

While this tab says Resource accounts, the accounts used for Microsoft Teams Meeting Room systems are not configured and managed here. Those are configured exclusively via PowerShell at the moment and are not included in the MS-300 exam. For more information on configuring Microsoft Teams Rooms system accounts, please see https://docs.microsoft.com/en-us/microsoftteams/room-systems/with-office-365.