This is when an ISP takes over a dormant/closed email account that could not have possibly opted in anywhere so anyone sending email to this address is likely sending unsolicited email.
The way the process works is that the email service provider reviews a list of email addresses that have been dormant for a long period of time and disables the account. They leave the account disabled for 6-12 months and then re-activate the address as a spam trap in an attempt to catch spammers. If they catch anyone sending to that email address, it's obvious that either the person purchased a list, used software to pull email addresses from websites, or are not performing the proper email list hygiene.
Another scenario that an ISP can use is when a domain becomes inactive, after a holding period of 6-12 months, hosting companies disable the domains email services and forward any emails sent to them to blacklists. It should be noted that with this scenario, the mailing isn't going to bounce so you're not going to be notified it's a bad address until you find your account has been blacklisted.
Some spam tracking services actually create fake websites to capture spammers. They place hundreds of emails on the website that are spam traps or spam honeypots. If they send to those email addresses, blacklists know the client either purchased non opt-in lists or pulled them from one of the fake websites.
Steps you should take to keep your contact list in good standing
Use a real time email correction service to help keep problematic email addresses from ever entering the database.
Remove hard bounces immediately. Feedback loops are great for this. Our system has an automatic process to aid in the changing of bad emails to the Held status.
Update opt-out file on a regular basis and honor your recipient's wishes to opt-out.
Clean out your database on a quarterly basis to remove dormant domains and email accounts.