钩子能给我解释一下吗？

[jinkeb]

function TestHookProc(Code: Integer; WParam: Longint;Msg:Longint): Longint;stdcall;
begin
if (Code = HC_ACTION) then
if PMsg(Msg)^.Message = WM_TestMessage then
begin
showMessage('已经截获该消息');
end;
Result := CallNextHookEx(HookHandle, Code, WParam, Longint(@Msg));
end;

[xgc521]

if (Code = HC_ACTION) then// Code是键盘消息的处理方式 HC_ACtion表示收到一个正常的击键消息
if PMsg(Msg)^.Message = WM_TestMessage then //判断消息类型
begin //如果是自己需要的消息就执行自己的代码
showMessage('已经截获该消息');
end;
//挂钩下一个挂钩函数（因为钩子是一个链）
Result := CallNextHookEx(HookHandle, Code, WParam, Longint(@Msg));