A QR code is a square pattern of black and white blocks that a phone's camera reads as a web address. Point the camera at it, tap the link that appears, and the page opens. At a shop counter it saves a customer from typing a long address into a phone while holding a coffee or a bag of shopping, which is the difference between joining now and meaning to join later.
Most phones read QR codes from the built-in camera app with no extra software. The code itself is only a link: it carries no information about the shop beyond the address it points to, and it never changes unless the link does.
In practice
Size and placement matter more than design. A rough rule is that a code scans comfortably from about ten times its own width. A code 6 centimetres wide on an A6 card by the till works from about 60 centimetres, which suits someone standing at the counter. A code in the window meant to be read from 2 metres away needs to be about 20 centimetres wide.
Put it where people wait, not only where they pay: beside the coffee machine, on the bread bench, at the end of the counter where orders are handed out. Print it with a plain line underneath saying what scanning does, for example "Scan to join our stamp card", because people hesitate to scan a code that does not say where it goes.
Check your printed codes now and then. Scammers have been known to stick their own codes over genuine ones in public places, sending people to a lookalike page, and a quick look at the counter card once a week is enough to catch it.
In Counter Club
Your join link and its QR code appear on the Loyalty card page once you save the card, as a plain image you can print at any size. The same link is on the Members page with a Copy link button for socials and signs. See setting the card.