It’s a theory I picked up from reading Tipping Point. The main idea is like this:
Suppose that a house in a neighborhood has a broken window, and nobody decides to fix it. Eventually people will start to think that the house is either neglected, or people doesn’t care about the condition of the house. This will lead to vandalism around the house that will spread to the neighboring house, ultimately attracting criminals turning the neighborhood into an unsafe one.
Moral of the story: serious problems might arise from problems which are seemingly unimportant at first. The simpler problem will eventually attract a slightly more serious problem, and the process snowballs until you get a really serious problem. Which is why you shouldn’t neglect any kind of problem, no matter how small it seems. Because who knows, in the future it may cause you a lot of trouble.