This service was created so that the end users can reset their own passwords to access Office 365. It can also be configured to support writeback to the on-premises AD. It's a simple process to activate.

Go to http://portal.azure.com and then to your Azure AD panel, and select the Password reset option.

The following screenshot shows the blade on the Azure Portal showing how to configure it:

You can also select the Authentication methods option as shown in the following screenshot:

If you have Azure AD Connect installed on-premises, you can also enable the writeback password, as shown in the following screenshot:

To learn more about self-service password reset, continue reading at https://docs.microsoft.com/en-us/azure/active-directory/active-directory-passwords-getting-started.