Loose coupling is a software design approach that implies every part of an application should know little information about the other parts. In traditional applications, if you write a GUI component, such as a button, it has to be possible to use it everywhere, for any application. As another example, if you develop a library to work with sound hardware, this also means you can use it with every application; the library is not limited to use in one kind of application. In microservices, however, loose coupling means that a microservice doesn't know anything about other microservices, or how many there are.
If you develop an application, you should write parts of it—microservices—as standalone applications. For example, a notification service that sends push notifications to mobile platforms won't know about CRM, accounting, or even the users. Is it possible to do this? Yes! You can write a microservice that uses a common protocol and interacts with other services via message passing. This is called a message-driven application.