In this callback you might:
a) Make a HTTP request in JS to a controller-action that returns data from your records. Your JS should then process the results and update the page for the user. Most commonly this data is returned as JSON.
b) Redirect to a URL with your QR result embedded in the URL. So if your QR code decodes to dogs, you might redirect to https://myapp.com/search?q=dogs to search for dogs, or if it decoded to a record id 100, you might redirect to https://myapp.com/blog/100. This can be as simple as