The availability heuristic is a concept that emerged from the work of Kahneman and Tversky in the early 1970's. The term basically describes our tendency to think that whatever is easiest for us to call should provide the best context for future predictions. As the name suggests, this effect exists because our brains have evolved to operate under the assumption that if a memory is easily recalled, then it must be important to the current context. In some cases, a memory may be available because it reflects something that happened to us recently. In other cases, the event may have occurred a long time ago but it somehow left a meaningful imprint on us. In either case, as long as the memory left is meaningful to us, our brain assumes that it is useful information for making future predictions.
A commonly cited example of the availability heuristic is Kahneman & Tversky's experiment that asked people to recall words that begin with the letter K versus those that have K as their third letter. Because we can much more easily recall words such as kitchen, kangaroo and kale, we ignore the fact that there are actually about twice as many words with K in third place (e.g. ask).