Must-Read Site

Signal-Slot-Konzept
Crash course in Qt for C++ developers, Part 3, Signals and slots – communication between object

Rest

The C++ preprocessor changes or removes the signals, slots, and emit keywords so that the compiler is presented with standard C++.
Qt Signals & Slots
Wikipedia Signal-Slot-Konzept
Wikibooks Qt für C++ Anfänger: Signale und Slots

class Zahl : public QObject 
{
    Q_OBJECT

private:
    int wert;
   
public:
    Zahl();

public slots:
    void setzeWert(int wert);
 
signals:
    void wertGeaendert(int neuerWert);
};

Tutorials

NICLAS ROSSBERGER

Qt Signals & Slots: How they work

woboq

How Qt Signals and Slots Work
How Qt Signals and Slots Work – Part 2 – Qt5 New Syntax
How Qt Signals and Slots Work – Part 3 – Queued and Inter Thread Connections
QMetaType knows your types