A representativeness heuristic is a cognitive bias in which an individual categorizes a situation based on a pattern of previous experiences or beliefs about the scenario. It can be useful when trying to make a quick decision but it can also be limiting because it leads to close-mindedness such as in stereotypes. There are several types of representative heuristics, including the Gambler's Fallacy, Base Rate Fallacy, Regression To The Mean, and Conjunction Fallacy.
Just to provide one example, let's look at the Gambler's Fallacy. This is a person's belief that the probability of an item changes based on previous attempts when in reality, the probability remains the same. If a coin was flipped 10 times, and each time it landed with the "heads" side facing up, someone relying on gambler's fallacy would believe the odds of it being heads the 11th time would be very low. In reality, however, the probability has not changed. The chances of a coin being heads or tails is 50% no matter how many times the coin is flipped.