This enables users to log in to all of their EC2 instances with a single set of domain credentials (no key pair needed) and set access controls, allowing domain admins to control which users can access particular instances.
Simple AD is a managed directory powered by a Samba 4, Active Directory Compatible Server. It supports commonly used features such as user accounts, group memberships, domain-joining Amazon EC2 instances running Linux and Microsoft Windows, as well as Kerberos based single sign-on (SSO), and Group Policies.
This makes it very easy for companies to be able to manage Amazon EC2 instances in the Amazon Web Services cloud.
Amazon have more details on their blog.