What does it mean that the availability heuristic?

You overestimate the likelihood of events you can easily recall examples of. The availability heuristic substitutes the question "how frequent is this?" with the question "how easily can I think of an example?" — and the substitution happens below conscious awareness, which means you feel like you are reasoning about probability when you are actually reasoning about the vividness of your memory.

Example: After watching three consecutive news segments about home invasions, you check the locks on your doors twice before bed and consider installing a security system. You have lived in the same neighborhood for six years without incident. The local crime rate has been declining steadily. Nothing in your environment has changed — but your perceived risk has spiked because vivid examples are now loaded in your memory. You are not responding to a change in actual danger. You are responding to a change in the ease with which danger-related scenarios come to mind. The news did not make your neighborhood less safe. It made threat scenarios more mentally available, and your brain treated that availability as evidence of frequency.

Try this: Pick a domain where you make frequency or probability judgments — your health, your finances, your career, crime in your area, risks to your children. Write down your intuitive estimate of how likely a specific negative event is (e.g., "chance of being burglarized this year," "chance of being laid off," "chance of a specific disease"). Then look up the actual base rate from a statistical source. Compare your intuitive estimate with the data. For any case where your estimate was significantly higher than the base rate, trace the source: can you identify a vivid example — a news story, a personal anecdote, a dramatic portrayal — that inflated your estimate? Document the gap between your felt probability and the actual frequency. This gap is the availability tax you are paying on that judgment.