What do you understand by module in Angular?

Module is the logical boundary in Angular Application where the application is further divided in separate modules in order to separate the functionality in application. It is basically a mechanism used to group the directives, pipes, components, and services, which can be further combined to other modules in order to create an application.
In order to define the module, NgModule decorator is used- 

A module can either be exported or hidden, where exported elements are being used by the other modules whereas the hidden ones are being used inside in the module which can’t be further accessed directly by the other modules.