What are Office 365 Groups?

Office 365 Groups are only available with Sharepoint Online, and serve as a collaboration tool

From a management perspective, you can think of an Office 365 Group like an Active Directory. 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. 

You can learn more about Office 365 Groups on the Microsoft help center.