Azure AD App registrations provides ways for authenticating or authorizing the user custom applications which uses Azure AD, to access the data available on the Microsoft products/services.
Why Azure AD App Registration? You might have already worked with custom applications that uses Azure Control Service, which provides way for authenticating users to gain access to web applications and services. As ACS is getting deprecated, Microsoft provides ways for authenticating the applications and provides way for enabling permissions for users to access the data.
The following method is one way for registering the custom built applications and authenticating users for accessing Microsoft services.
1. Azure AD has options for registering the user custom applications, where users can provide the app details like app name, application type (web or native app) and sign-on page URL. This helps authenticating users for accessing the application data.
2. Then navigate to the required permissions option. Select the required API and add the necessary permissions and save it. After saving, click on grant permissions for enabling users to access the data.
3. Navigate to the reply URLs option and add your custom application URLs. The reply URLs are used as callbacks. The tokens from the Azure AD will be passed to the applications using query parameters in the reply URLs.
On Azure AD, necessary permissions are given. Next, the custom applications can initiate the calls for authentications and access the data available on Microsoft services like graph API, outlook, calendars, etc. The following post describes one such authentication method to access the calendar events of user from outlook using graph API and outlook API.
The following Microsoft article describes more about authentication mechanisms using Azure AD.