The Matthew Effect is essentially the idea that the rich tend to get richer, while the poor tend to get poorer. It comes from the Gospel of Matthew in the Bible, which reads, "For to every one who has will more be given, and he will have abundance; but from him who has not, even what he has will be taken away" (25:29). In the context of the Bible, this verse means that those who work for God's grace have increased opportunities to find even greater grace. Those who do not have grace will not have opportunities for more grace.
Gladwell uses the Matthew Effect in Outliers to explain why children, for example, who show some early talent in some area, such as academics or sports, receive more and more help and encouragement from their teachers and coaches, while those who do not initially show this promise do not always receive encouragement. Therefore, those who are promising at a skill tend to get better and better, while those who are not initially promising do not have as many chances to improve. There is a great advantage to showing early promise or having an early advantage.