This is useful when exactly one object is needed to coordinate actions across the system. The concept is sometimes generalized to systems that operate more efficiently when only one object exists, or that restrict the instantiation to a certain number of objects.

For more information please refer the following link.

cronj.com/blog/design-patterns-javascript

In simple words, Singleton means

Assume u having an object but it is injected in many places, but it only uses object reference, In AngularJS all services are singletons. service, providers, factory, constants all are a singleton.

Eg: If you wanna access a data from one controller to another controller singleton service is used. Refer this for clear idea on service & factory.