Do you have an iPhone? Then you can scan the code through your Camera app.
Don't take a picture of the code, but bring the code into view. Within a few seconds, your smartphone will recognize that it's a QR code.
Don't have an iPhone? Then you probably need a separate app to scan the code. Via Google Play you will find several scanner apps that are free. Firefox for Android, for example, has a built-in QR code scanner.
Scanning QR codes can also be done via the Antwerp UCoins web application after login. The payment function is accessible from the home screen.