The basic idea is that you take a picture of the old printed photo with your smartphone and save it forever in the digital cloud. Of course, if you try to do that normally, you'll run into troubles like light reflecting off the photo, your own body casting a shadow over part of the image, or even just not getting the image to come out straight. PhotoScan uses the power of Google to automatically detect edges, straighten the image, rotate it to the correct orientation, and remove glare.
In practice, it works by taking five shots of each photo and stitching them all together to give you the best representation of the old photo you can get. Add that to the more advanced new photo editing tools that were added to Google Photos and you should be able to put together a pretty awesome family album on the web and share it to your loved ones.
source: Google Photos