Well, after enabling this new setting, these attendees will be able to join the meeting in ‘view-only’ mode.
This means they will not be able to share audio/video, see or participate in the meeting chat or see the video feed from other participant – except the presenter, or see PowerPoint presentation shared using the native PowerPoint feature.
In addition, if a policy managing the ‘bypass lobby’ setting, they may not be able to join if they don’t have the permission to bypass the lobby and they will not be able to join the ‘Overflow Room’.
If you want to enable this capability, you need to run the below command; the setting is disabled by default
Set-CsTeamsMeetingPolicy -Identity Global -StreamingAttendeeMode Enabled