Active Lurking, also referred to as Fluffposting or Fillering, is the act of posting (thus differentiating it from ordinary Lurking), but the material posted is irrelevant or otherwise useless for scumhunting.
Examples of active lurking include posts made only of taunts, excuses for not posting, incoherent gibberish that will lead people to suspect that you do not have the Internet savvy to play Mafia, general bland agreement with whatever is going on, prod dodges, and so forth. For the less couth readers, this is frequently called "bullshitting" in MeatWorld.
This tactic is employed by scum who wish to appear more active than they actually are; either for the scum who has no comment on the current matter but does not want to seem like they are lurking, or for the lurker who wants to paint themselves as slightly better than some other lurker.
The effectiveness of this tactic is quite impressive if players are not specifically looking for it. It is a common Town mistake to wish for the lynch of people they violently disagree with over the people who post whatever they can to stay on the periphery. However, Towns nowadays are often aware of this behaviour and will consider it a major scumtell.