"Concerted cultivation" is a parenting style in which the parents structure their children's time and expose them to different situations, often involving them in clubs and teams. (The other style of parenting is known as "accomplishment of natural growth," which essentially allows children to determine their own entertainments; it is largely a hands-off parenting style.) Gladwell observes that children who are raised with "concerted cultivation" tend to be better prepared for higher learning and better white-collar jobs, as they've learned very young how to behave and be comfortable with different groups of people, whereas the "natural growth" children were less prepared. 

Gladwell's thesis is that what we become is largely determined by birth (place and time of birth, socio-economic status, race, etc), the link here is that middle- and upper-class children are likely to be objects of "concerted cultivation" while lower-class children are left to develop on their own, giving the middle- and upper-class children another invisible "edge" on the lower-class children later in life.