QR codes are those little boxes of squiggly lines that you see in stores or on the subway or bus that you can take a picture of with your Smartphone or Ipad using a QR Code reader from the app store. You can embed information in them such as contact info or a link to a website or a survey or something. They are extremely useful in Korea because almost everybody has a smartphone with a camera and in a class of 40 students, there's probably a 99% chance that every single student has one.
I use them in situations in class where I want the students to go to a website on their smartphone, but it's not an easy link for them to just type in (ie: a Google Doc file). On my PPT, I'll post up an image of my QR code that I've made using a QR code Generator. The students just scan it and then they can have the link to the Google Doc that I want them to use or look at. Here is the PPT from my next week's Advanced English Composition class, where we're focusing on editing their essays that they've done for homework. If you scroll down to slides 7 and 12, you see can examples of what I've done.