QR is short for "Quick Response". A QR Code is a barcode that contains data that can be read by a phone's camera. 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.
Unlike other barcodes which have to be sent to a database to retrieve information, QR codes are self-contained; hence the "Quick Response."
Most smartphones have a tool 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.
To scan a QR code you will need:
In the same way a "phishing" email can lure you to visit an unsafe website or convince you to share personal information which is then used maliciously, QR codes can lead to harm if you don't take precautions.
More tips and details are available below, but, at a minimum, you should pause and consider whether you can trust the source of the code before you scan it. Is the code in a reputable publication, on a trusted website, or on a legitimate poster or display that has not been tampered with? If you're not sure, it's probably best to avoid scanning it unless you take further precautions.
This article (link above) summarizes the risks and suggests additional steps to assure a safe scanning experience.