Blacklists are real-time lists containing email addresses and/or IP addresses that are known for sending SPAM.
If the IP address for your web server is on a blacklist maintained by an organization such as Spamhaus, Barracuda, Invaluement, LashBack, Passive Spam Block List (PSBL) or SORBS, you may experience delivery problems with emails (eg: user registration emails, password reset emails, etc.) sent from your server.
You can request removal of your email and/or IP address from a blacklist and each of the organizations mentioned above provide a process for making such requests.
Companies such as AWS and DigitalOcean re-distribute IP addresses once they’ve been released by their previous owners. If you’ve just set up a new web server and are getting ready to install a whole bunch of software, it probably makes sense to first check and see if your new IP address is on any blacklists.