Universal grammar consists of a set of unconscious constraints that let us decide whether a sentence is correctly formed. (The mental grammar is not necessarily the same across Languages).
According to Chomskian theories, the process by which certain sentences are perceived as correct (or incorrect) is universal and independent of meaning.
Traditionally, Language learning was believed to be similar to any other kind of learning: explained by a succession of trial and error. Children learn their mother tongue by imitating, listening to and repeating what adults say.
Chomsky argued against this theory. His view was that Language cannot be reduced to the development of responses to stimuli, because every sentence that anyone produces can be a totally new combination of words.
Speech can be thought of as the process of combining a finite number of elements (the words) to create an infinite number of larger structures (sentences).
Chomsky noted that children can (without any formal instruction) consistently produce and interpret sentences that they have never encountered before. He believed that the reason that children master complex operations of Language so easily is due to their innate Knowledge of certain principles which guide them in developing the grammar of their Language. (Language learning being facilitated by a predisposition that our brains have for certain structures of Language).