What is declaration in ngmodule ?

Declarations are used to define components, directives, pipes in the module. It can be used in the another component but in the same module.

For example:

Firstly you have a main app component which is also our root component, so likewise if we create any other components like server, login, etc. We need to define it in the @ngmodule declaration part which is present in the app.module.ts file.