This tool works by breaking the country down into postcode sectors and then recording the crime rates of the sectors that are amongst the lowest top third.
For example, if you are thinking - I wonder where there is low crime near me? then you would enter your current location in the location box, set a radius to search within and start the search. The tool would then return all areas within that radius that have crime rates comparable to the lowest top third crime rate areas.
There are many reasons why you would want to do a search like this aside from just as a crime rate checker.
Crime is one of the top ranking factors that go into what make somewhere a good place to live against a bad place to live. Some of the nicest places to live are within the top third of lowest crime rates.