Organisations in Re:infer can be thought of as restricted workspaces. Sources and Datasets are both linked to specific Organisations when they are created, and users must be part of an Organisation (and have the correct user permissions) to view any of the data linked to it.
Your company may have multiple different Organisations associated with it, based on projects, teams, business units or data restrictions. Users can be given access to multiple different Organisations that are linked to their company.
To understand more about Organisations, see here.