I wanted to understand the purpose of this code snippet

Can someone please explain the purpose of this C# code. It's a code snippet in a Windows based application. Is it counting the number of keypress? What is the purpose of 13 here?

Any help would be much appreciated

 private void num2_KeyPress(object sender, KeyPressEventArgs e)
    {
      if ((int) e.KeyChar != 13)
        return;
      this.Calculate();
    }