The myth is that a trial or event in gambling that has not recently happened becomes overdue and thus becomes more likely to occur.
Suppose that a person tosses a coin 6 times and gets a head each time. If one concludes that the next toss will be tails because tails "is due", then he will have committed the Gambler's Fallacy.
What Does Gambler's Fallacy Mean?
When an individual erroneously believes that the onset of a certain random event is less likely to happen following an event or a series of events.
This line of thinking is incorrect because past events do
not change the probability that certain events will occur in the
future.
Investopedia explains
the Gambler's Fallacy
For example, consider a series of 20 coin flips that have all landed
with the "heads" side up. Under the gambler's fallacy, a person
might predict that the next coin flip is more likely to land with
the "tails" side up.
This line of thinking represents an inaccurate understanding of probability because the likelihood of a fair coin turning up heads is always 50%. Each coin flip is an independent event, which means that any and all previous flips have no bearing on future flips
Here is a link to a nice article explaining the gambler's fallacy. The site shows an example of coin tosses in a row and why after 19 in a row the 20th toss still is a 50-50 bet.
The smart gambler would wait until tails finally showed, say after 22 heads in a row, and then bet against the next 22 flips that heads will NOT show again 22 times in a row. Of course that is easier said than done!