To build apps that use the Microsoft identity platform for identity and access management, you need access to an Azure Active Directory (Azure AD) tenant. It's in the Azure AD tenant that you register and manage your apps, configure their access to data in Microsoft 365 and other web APIs, and enable features like Conditional Access.
A tenant represents an organization. It's a dedicated instance of Azure AD that an organization or app developer receives at the beginning of a relationship with Microsoft. That relationship could start with signing up for Azure, Microsoft Intune, or Microsoft 365, for example.
Each Azure AD tenant is distinct and separate from other Azure AD tenants. It has its own representation of work and school identities, consumer identities (if it's an Azure AD B2C tenant), and app registrations. An app registration inside your tenant can allow authentications only from accounts within your tenant or all tenants.
To check the tenant:
If you don't have a tenant associated with your account, you'll see a GUID under your account name. You won't be able to do actions like registering apps until you create an Azure AD tenant.
You'll provide the following information to create your new tenant:
<domainname>.onmicrosoft.com
can't be edited or deleted. You can add a customized domain name later.