The availability heuristic refers to the tendency of the human mind to assume that events that spring easily to mind happen with equal frequency in the real world. Heuristics are cognitive shortcuts used by the brain to navigate the world more efficiently.
Changing Minds explains that the availability heuristic is an often useful algorithm of the human mind that aids in determining frequency and probability. Psychology and Society gives a practical example of the availability heuristic at work in everyday life. Those who have seen stories about foreclosed homes on the news with great frequency are far more likely to judge foreclosure as an event that happens more often in real life than it actually does. Likewise, the prevalence of articles written about road rage incidents lead people to believe that the phenomenon is far more common than it really is according to statistics.
The availability heuristic has some use in preparing the mind to expect an event it has had significant exposure to, but like all heuristics, it can lead to bias if left unchecked. The human mind naturally attempts to process information in the most efficient way possible, so heuristics act as general rules that give the brain a formed conclusion before all facts about the individual system are reviewed.