What is meant by the “Barnum effect”? Why is this a source of concern for many

The Barnum effect refers to the gullibility of humans when they are offered such things as horoscopes, handwriting analysis, and results of psychological tests.
It is named after the great circus showman, P. T. Barnum, who knew that the formula for success was to “have a little something for everybody.”
He also said, “There’s a sucker born every minute.”
There are three guidelines to follow to avoid being taken in by unscientific personality profiles and horoscopes:
Beware of all-purpose descriptions that could apply to anyone.
Beware of your own selective perceptions.
Resist flattery.