“Digital footprint” does not refer to any specific thing. It’s a catch-all term to describe whatever information about you exists out in the world. Different companies (and different people) will have different information. For example, let’s say you’ve never used the internet before and sign up with an ISP. Your ISP now knows your address and billing information. Maybe they ran a credit check. Now you hook up your computer and fire up Google. If they want to, your ISP can figure out what kind of computer you have. They know you visited Google. Google knows your IP address and browser type but not (yet) who you are. You type “reddit” into a Google search. Now Google knows you are interested in Reddit. Click on that Reddit link. Now your ISP and Google know you visited Reddit. Reddit knows your IP and browser type. And so on as you sign up for Reddit, visit other sites, etc. Each step you take leaves a tiny bit of information about you. Collectively, all that information is your digital footprint. Once you give one of those companies your name, now your name is associated with your digital footprint. Where things get interesting (and problematic) is when a company starts correlating the different bits of information you leave on different sites. Maybe those companies sell your data or maybe they use cross-site trackers from Google or one of the big advertising companies. By collecting all those little bits here and there, big data aggregators can build up a surprisingly detailed and accurate picture of who you are. Maybe you gave one site your email address but not your name. Another site knows your name but not your email, etc.
The single most important thing to know about privacy and security is perfection does not exist— never has and never will. So, our work is to do the best we can with the resources we’ve got and understand that there is always some work left over. You can never totally erase your digital footprint. In some cases you can erase parts of the footprint but that is a lot of work. What you can do is leave a smaller footprint from now on.
How to map your footprint
How to leave no trace
Never raw-dog the internet
- Use a trusted VPN provider 
- You can also chain Tor -> VPN with Proxychains if you want to not be fully trusting on your VPN
Sign up for email from Tor / VPN, never log-in with real IP
Protonmail has a .onion but does not allow sign ups from it
Tutanota will allow sign ups from Tor but it has a waiting period after account creation
Use Email Aliases
Protip: Firefox Aliases can be used to sign up for SimpleLogin Aliases and vice-versa, you can get 100s of free Aliases this way.
Use Number Aliases
https://jmp.chat/ - JMP.Chat - Private VoIP number with handful of number aliases
https://crypton.sh - Crypton.sh - Private Physical SIM number. XMR accepted
https://juicysms.com/ - JuicySMS - For SMS verification, different number is given each time which helps keep accounts unrelated. XMR accepted.
Avoid Non-Private Services Whenever You Can
How to clear historical tracks
- Purge (edit content to null) and abandon accounts, don't delete them. Provider has no obligation to fully delete your account or the information stored within, they instead often become a private account you no longer retain access for.