What is Office 365 or Azure AD tenant?
In Office 365 or Azure Active Directory (Azure AD), a tenant is representative of an organization. It is a dedicated instance of the Azure AD service that an organization receives and owns when it creates a relationship with Microsoft, such as by signing up for a Microsoft cloud service like Azure, Microsoft Intune, or Office 365. Each Azure AD tenant is distinct, unique and separate from other Azure AD tenants.
A tenant name cannot be changed, so choose your name wisely.
A tenant houses the users in a company and the information about them—their passwords, user profile data, permissions, and so on. It also contains groups, applications, and other information pertaining to an organization and its security.