C++ keyboard hook - what does the parameter nCode mean?

I am looking to write a keyboard hook for a c++ project. I found some code but dont want to use it without fully understanding it:

HHOOK _hook;
KBDLLHOOKSTRUCT kbdStruct;
LRESULT __stdcall HookCallback(int ncode, WPARAM wParam, LPARAM lparam)
{
    if(ncode>=HC_ACTION)
    {
        if((wParam == WM_KEYDOWN) || (wParam == WM_SYSKEYDOWN))
        {
            kbdStruct = *((KBDLLHOOKSTRUCT*)lParam);
            DWORD dwMsg = 1;
            dwMsg += kbdStruct.scanCode << 16;
            dwMsg += kbdStruct.flags << 24;
            char key[16];
            GetKeyNameText(dwMsg,key,15);
            
            if((GetKeyNameState(VK_CAPITAL)& 0x0001) == 0)
            {
                for (int i=0; i<10)key[i] = tolower(key[i]);
                ReturnKeyPressed(key);
            }
            else
            {
                ReturnKeyPressed(key);
            }
        }
    }
    return CallNextHookEx(_hook,nCode,wParam,lParam);
}
void SetHook()
{
    _hook = SetWindowsHookEx(WH__KEYBOARD_LL,HookCallback,Null,0);
}

I dont understand what nCode is here. And where do the other parameters come from? Greeting from an absolute C++ beginner :).

In a nutshell, nCode tells you whether the wParam and lParam contain valid data or not. If nCode is HC_ACTION (0), then they do, otherwise they do not. This is clearly stated in the documentation:

• nCode [in]
  Type: int

  A code the hook procedure uses to determine how to process the message. If nCode is less than zero, the hook procedure must pass the message to the CallNextHookEx function without further processing and should return the value returned by CallNextHookEx. This parameter can be one of the following values.

  HC_ACTION
  0

  The wParam and lParam parameters contain information about a keyboard message.

• wParam [in]
  Type: WPARAM

  The identifier of the keyboard message. This parameter can be one of the following messages: WM_KEYDOWN, WM_KEYUP, WM_SYSKEYDOWN, or WM_SYSKEYUP.

• lParam [in]
  Type: LPARAM

  A pointer to a KBDLLHOOKSTRUCT structure.