What is meant by the "broken windows thesis"?

Proposed by Wilson and Kelling, the Broken Windows Thesis is the theory in Crime and Deviance that crime thrives in areas that appear to already have a substantial amount of crime go unchecked. For example, an area with broken windows and a large amount of graffiti is likely to have an increased amount of crime compared to an area without such aesthetic ailments because those who live there begin to feel as though their area is not valued to the same degree as others by those above them such as the government and the police. Therefore, behaviour naturally spirals downwards and begins to thrive.
This is why local councils and charities try and ensure that any anti-social behaviour such as graffiti and vandalism is dealt with as soon as possible to avoid people beginning to feel as though such actions are okay to partake in.