The Broken window effect is based on a theory espoused in the book Fixing Broken Windows, that minor signs of disrepair in a community can lead to people assuming that nobody cares, and to a snowballing effect of negligence, vandalism, and bad behaviour.
This metaphor is often used to describe why communities should be vigilant about tasks like Spam control, Wiki gardening, and other such administrative tasks.
(Note that there has been considerable criticism of the original theory regarding urban decay and crime, but for the most part this doesn't affect the metaphorical use of the term in community management.)