What do scientists mean when they say that there are critical periods for brain development? why are these periods important?

A critical period is stage of maturation in an organism's lifespan during which the nervous system is sensitive to particular environmental stimuli. This period is very important because this would help or aide in an organism's survival. If, for some reason, an organism fails to receive the appropriate stimulus during this "critical period" it would be unlikely for the organism to learn a skill or trait.