How the below code works

Question

Hi

  How the below code workd

openDialog($event:Event){
    this.openModal.emit($event);

  }

<div class="m-l-10" matTooltip="ADD">
                  <button mat-mini-fab color="primary" (click)="openDialog($event)">
                    <mat-icon class="col-white">add</mat-icon>
                  </button>
                </div>

Thanks