Hi Dhairya

Organisations are able to be created in hierarchies and are assigned to a user through their job assignment. These are site wide structures so available to all tenants, audiences and groups.

Groups are also subsets of users aimed at keeping users in course activities apart

Audiences are subsets of users across the site that allows common procedures such as enrolling users to be actioned.

Tenants separates users for all aspects of Totara - courses, activities, performance activities and allows separate themes and roles for each tenant

So basically they are way to separate content and users at different levels with organisations available at all levels.

Regards