Digital Footprint means the mark you leave on the internet. You leave one every time you send an email, post a video, or make a post on facebook or twitter. When you do these things, it leaves a trail of you on the internet. This can be tracked so you need to make sure no one with bad motives tracks you down.
It is important to leave a positive digital footprint. If you don't, people that see what you post may be offended. Some people have the skills to track that post to you. If they do, it could put you in danger.
Some ways to keep track of your digital footprint are: Google's Me On The Web, vanity searches, and Google alerts which notifies you every time your name is used on the web.