In ActivityInfo, a Partner is an organization who will interact with your Database. Usually, a Partner will be an organization such as an NGO, private company or government organization. Partners are used in the template of the Multi-partner reporting database.
By using Partners, you can link the users you add with the organizations they belong to. This will allow you greater control over what data a user can view or edit for example.
Using Partners gives you an extra layer of detail when you report on your data, allowing you to filter and group entries performed by users of a specific Partner.