You overestimate the probability of something that you hear more often/remember easily. Eg. We think accidents kill more people than strokes.
The availability heuristic operates on the notion that if something can be recalled, it must be important, or at least more important than alternative solutions which are not as readily recalled. Some researchers argue that the number of examples recalled from memory is used to infer the frequency with which such instances occur. Some others proposed the ease of retrieval explanation, in which the ease with which examples come to mind, not the number of examples, is used to infer the frequency.
People tend to heavily weigh their judgments toward more recent information, making new opinions biased toward that latest news. Media coverage can help fuel a person's example bias with widespread and extensive coverage of unusual events.
It is a System 1 mental shortcut.