QR codes are 2D barcodes that can be scanned by mobile devices to access URLs, audio/video files, texts, and Google Forms. The document discusses how to create QR codes using various free online tools and apps, and provides tips on using QR codes in the classroom for activities like treasure hunts, polls, distance learning, and linking to presentations. QR codes have educational potential to engage students and provide quick access to digital content from their mobile devices.