app.module.ts file

import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';

import { AppComponent } from './app.component';
import { ChildComponent } from './child/child.component';

@NgModule({
  declarations: [
    AppComponent,
    ChildComponent
  ],
  imports: [
    BrowserModule
  ],
  providers: [],
  bootstrap: [AppComponent]
})
export class AppModule { }

app.component.html file

<h2>Parent Component</h2>
<app-child [loggedIn]="userLoggedIn"></app-child>

app.component.ts file

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

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css']
})
export class AppComponent {
  userLoggedIn = true;
}

child.component.html file

<h3>Child Component</h3>
<p *ngIf="loginFlag">Welcome back</p>
<p *ngIf="!loginFlag">Please be login</p>

child.component.ts file

import { Component, OnInit, Input } from '@angular/core';

@Component({
  selector: 'app-child',
  templateUrl: './child.component.html',
  styleUrls: ['./child.component.css']
})
export class ChildComponent implements OnInit {

  @Input('loggedIn') loginFlag: boolean;

  constructor() { }

  ngOnInit() {
  }

}