A Microsoft 365 Group is an Azure Active Directory object that allows you to manage the membership of a number of workloads like shared Exchange mailboxes, SharePoint, and Teams.
It is a feature that allows you to control access (through assigning roles and allowing/denying access requests from non-group users) to other services like SharePoint sites, Teams, Planner, and so on.
For more information, see The relationship between Microsoft 365 Groups, Teams, and other services.