Microsoft 365 Groups lets you choose a set of people that you want to collaborate with and allows you to easily set up a collection of resources for those people to share. You don't have to worry about manually assigning permissions to all those resources because adding members to the group automatically gives them the permissions they need so they can access the tools your group provides.
Groups are the new and improved experience for what distribution lists or shared mailboxes used to do.
More questions? Check out Learn about Microsoft 365 Groups.