how does the qr reader widget work?

Which QR widget are your referring to? Please check them in our widgets page: https://widgets.jotform.com/search/qr 

Click in any of them, and you will see the description of how they work.

If you do not know how to add them to your form, please check this guide:

https://www.jotform.com/help/252-How-to-Add-a-Widget-to-Your-Form 

Let us know if you have more questions.