Spaces are a place to organize your activities and include others. Use a space to get things done with project teams, social clubs, groups of friends, or to organize your personal life. People invited to this space can collaborate on tasks, events, documents, and through conversations with you. Others will only be able to see spaces you have in common with them.
So for example, if you have a Book Club Space with Jane and Ted, and a Running Team Space with Sarah and Bill, Jane will not be able to see anything related to the Running Team and Sarah will have no visibility into your Book Club.