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What is Injectable decorator?

Marks a class as available to Injector for creation.

import { Injectable } from '@angular/core';

@Injectable({
  providedIn: 'root',
})
export class UserService {
}

The service itself is a class that the CLI generated and that's decorated with @Injectable().

     What exactly does providedIn do?

Determines which injectors will provide the injectable, by either associating it with an @NgModule or other InjectorType, or by specifying that this injectable should be provided in the 'root' injector, which will be the application-level injector in most apps.

providedIn: Type<any> | 'root' | null

     providedIn: 'root'

When you provide the service at the root level, Angular creates a single, shared instance of service and injects it into any class that asks for it. Registering the provider in the @Injectable() metadata also allows Angular to optimize an app by removing the service from the compiled app if it isn't used.

     providedIn: Module

It's also possible to specify that a service should be provided in a particular @NgModule. For example, if you don't want a service to be available to applications unless they import a module you've created, you can specify that the service should be provided in the module

import { Injectable } from '@angular/core';

import { UserModule } from './user.module';

@Injectable({
  providedIn: UserModule,
})
export class UserService {
}

This method is preferred because it enables Tree-shaking (Tree shaking is a step in a build process that removes unused code from a code base) of the service if nothing injects it.

If it's not possible to specify in the service which module should provide it, you can also declare a provider for the service within the module:

import { NgModule } from '@angular/core';
import { UserService } from './user.service';

@NgModule({
  providers: [UserService],
})
export class UserModule {
}