The title says it all, but I’ll go into more detail: When you see a QR code in public, use your cellphone’s camera on it.
If you open your camera and point it at a QR code, the camera software can “figure it out.” Watch for a clickable link on your screen (you do not need to actually take a picture). If you tap the link that appears, it will do whatever the QR code is programmed for (take you to a website, start an email, etc.).
Do NOT download and install any 3rd party app for QR code reading. Those apps are wholly unnecessary at this point, and can contain adware or worse.