The orgs (short for organizations) module contains models related to the organization (what can be thought of as workspace) for a set of particular RapidPro users. Since RapidPro is a multi-tenant system, Organizations act as a way of tying together a set of channels, contacts, messages etc..
The Org model captures basic attributes around an organization, including the name of the organization, the languages it uses, locale and timezone as well as what users are granted permissions to view, edit and administer the organization.
TopUps represent the number of credits that have been added to an organization. Since RapidPro has roots in being a SaaS platform, TopUps are used as a way of monetizing use. The granting of TopUps via purchases is left up to service providers to implement.