What is meant by the notion of a critical period for language acquisition?

In childhood for many organisms, there is a "critical period," which is when important learning of certain skills occurs.

The critical period of language learning is a period in the first few years that is crucial for learning a language (language acquisition). After this period, it becomes harder and harder to learn a new language. This is why some immigrants that immigrate later in their lifetime struggle to completely grasp a new language.