Unique is a library for writing single-instance applications. If you launch a single-instance application twice, the second instance will either just quit or will send a message to the running instance. Unique makes it easy to write this kind of application by providing a base class, taking care of all the IPC machinery needed to send messages to a running instance, and also handling startup notification.
This module is heading towards planned deprecation. It will continue to be supported and API/ABI stable throughout the GNOME 2.x series, but we do not recommend using it in new applications unless you require functionality that has not already been moved elsewhere.