Basically a users comments and submissions are automatically killed (you can see them by turning on 'showdead' in your profile) to everyone but the user.

It's a very useful technique for fighting spam and trolls because it hides their activity from the rest of the site, yet they don't know that there is a problem so they don't work around it or create a new account.

The HN mods like to use hellbans on trolls, however it can sometimes be hard to distinguish a troll from a misguided or obnoxious user. Also, sometimes an otherwise good user will have a thread where they come off as trolling for one reason or another. In these cases it can be hard to know whether a user should be banned or not.