A QR Code (short for "Quick Response") is a barcode that contains data that can be read by a phone's camera. Most smartphones come equipped with software that allows them to download a QR Code reader. Once you download a reader, the phone's camera acts like a scanner, allowing it to "read" the barcode.
Since the June 2019 update patch, Android phones running Android 8 (Oreo) and above can scan QR Codes using the phone's native camera app. Apple phones have this camera functionality as well.
These codes, once scanned by your phone, can provide you with a URL, contact information, SMS, or similar links to information right on your phone. Think of it as a black and white link, or a heavily pixelated one.