The crux of the issue is that while confidence can be a very good thing, people are also susceptible to overestimating their skills. This may lead to a false sense of confidence, and consequently, under-preparation for a test (such as the SAT exam or ACT exam), or for a performance of some type.
Ms. Paul provides some suggestions for avoiding the "Overconfidence trap," such as delaying your self-testing of material just studied, in order to determine whether it has been retained, and putting your books aside while you self-test.
Another suggestion is to challenge yourself by testing yourself with material in a different sequence than the way it was originally presented, or by asking yourself to answer questions from a different angle than what you have typically studied. An example would be to solve for a different variable in a math problem, or to start with definitions of words and to look for the matching word. Ms. Paul provides links to research studies that confirm the efficacy of these suggestions.
The most important suggestion of all is to be aware of the tendency towards overconfidence. Being mindful of the need to thoroughly learn, and to test oneself, in appropriate ways, can mean the difference between overconfidence and appropriate confidence (which can help with performance).