Although it is a little difficult word, “fallacy” is a mistake in thoughts and knowledge.
Gamblers sometimes make irrational decisions, such as "Prices have been declining so far, so they should rise this time."
For example, in the case of a coin toss, even if the back side appears 10 times in a row, the probability that the front side will appear the 11th time is “1/2”.
But when you fall into the gambler’s fallacy, you assume that your trends will affect your next outcome. It is believed that these irrational behaviors affect the market.