This one is just as it sounds...a critical period really is a specific time during which an organism has to experience stimuli in order to progress through developmental stages properly. Have you ever taken a language class in school? Did you find it difficult? One reason is that we, as humans, have a critical period for language development. During that time, we are much more capable of learning a new language than at other, later times -- it just comes much easier during that time. In particular, if children don't start speaking around a certain time, it can become even more difficult for them to pick up their native language. Thus, there are critical periods for learning language. A related term is Sensitive Period.