It means that people tend to think that a random system won't ever act like a predictable system, when the nature of randomness is to occasionally seem predictable.
For example, in 100 coin flips, the odds are startlingly high (somewhere on the order of 25-to-one) that there will be at least five flips IN A ROW that have the same result.
But most people would never write "T" five times or more in a row if they were told to write down the results of an imaginary coin flip, because most people overestimate the uniformity of randomness.
