What a QR code can open
A website link, plain text, a Wi-Fi network (scanning connects the phone without typing the password) or a new email with the address and subject filled in.
Printing QR codes
Keep strong contrast — dark code on a light background — and leave the quiet margin around it. A higher error-correction level lets the code survive scratches or a small logo on top, at the cost of a denser pattern. Always test-scan before printing.