difference between onClick={() => this.func()} and onClick={this.func} in React js

I am a beginner to React and I tried onClick in these two ways. Can anyone explain the difference between these two?

1st way:-- onClick={() => this.func()}

func(){
  alert("Button Clicked");
}

render() {
  return (
      <div className="App">
        <button onClick={() => this.func()}> Click Me </button>
      </div>
  );
}

2nd way:-- onClick={this.func}

func(){
  alert("Button Clicked");
}

render() {
  return (
      <div className="App">
        <button onClick={this.func}> Click Me </button>
      </div>
  );
}