解释下代码..HOOK的

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs;

type
  TForm1 = class(TForm)
    procedure FormCreate(Sender: TObject);
    procedure FormDestroy(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}
var
  hook:Hhook;
function  Keyhook(nCode: Integer;wParam:WPARAM;lParam:Lparam):LRESULT;stdcall;
begin
   beep;
   Result:=CallNextHookEx(hook,nCode,wParam,lParam);
end;
procedure TForm1.FormCreate(Sender: TObject);
begin
    hook := SetWindowsHookEx(WH_KEYBOARD,Addr(Keyhook),HInstance,GetCurrentThreadID);

end;

procedure TForm1.FormDestroy(Sender: TObject);
begin
   UnhookWindowsHookEx(hook);
end;

end.
我运行的时候没有出现错误,小弟我正在练习HOOK,希望高手给小弟讲一下代码是什么意思?