Released to public preview on December 6, Azure AD pass-through authentication provides password validation for Azure AD services is performed against on-premises Active Directory, without the need for complex network infrastructure or for the on-premises passwords to exist in the cloud in any form. When combined with Single Sign on (public preview), users also need not type their password to log in to Azure AD or other cloud services, providing these customers with a truly integrated experience on their corporate machines.
Pass-through authentication is configured via Azure AD Connect and utilizes a simple on-premises agent that listens for password validation requests. The agent can be easily deployed to multiple machines to provide high availability and load balancing.
For more information you can review the following documentation: