Adolescence is a developmental period characterized by suboptimal decisions and actions that are associated with an increased incidence of unintentional injuries, violence, substance abuse, unintended pregnancy, and sexually transmitted diseases. Traditional neurobiological and cognitive explanations for adolescent behavior have failed to account for the nonlinear changes in behavior observed during adolescence, relative to both childhood and adulthood. This review provides a biologically plausible model of the neural mechanisms underlying these nonlinear changes in behavior. We provide evidence from recent human brain imaging and animal studies that there is a heightened responsiveness to incentives and socioemotional contexts during this time, when impulse control is still relatively immature. These findings suggest differential development of bottom-up limbic systems, implicated in incentive and emotional processing, to top-down control systems during adolescence as compared to childhood and adulthood. This developmental pattern may be exacerbated in those adolescents prone to emotional reactivity, increasing the likelihood of poor outcomes.

Okay here we go. "Suboptimal decisions" basically means bad decisions, decisions that have negative outcomes, choices that you would probably not make in the first place if you had known what would happen. Basically the first sentence is just saying that teens are more likely to make decisions that result in them getting hurt, getting into fights, or getting pregnant on accident. Next it's saying that the traditional explanations, based on brain chemistry or altered thought processes, do not do a good job of explaining why teenagers are like this. It says that new data from brain scans shows that teenagers act the way they do because they respond to rewards differently than adults do, and are more sensitive to the thoughts and judgment of their peers.

And there's some stuff about limbic systems and control systems that I think you'd need to actually read this study to understand.