It means that the IP address the host is using has been used for spam. If you have a server that has 100 websites and 1 site decides to spam people and they get reported to a DSBL type site all the sites on that IP address will be on the blacklist. Typically you can get clear from the blacklist if you pass the open relay tests on dsbl.org. But you will still be on the blacklist history list.
For more info:
http://dsbl.org/main
imaginemn