Our digital footprint is all the information we leave behind when using the internet. Credit/Debit cards, connecting to wireless internet, signing online petitions, posting on social media and browsing the web are all ways we can leave traces of our information (our digital footprint).
An example. Posting a photo of a child on their first day of school, mentioning what grade they are in. This provides so much information. This post provides your child’s (rough) age and shows what school they are in. All information, which you have unknowingly provided, with a single image.