Question - What Does Mean The Language Is Closed?
Answer -
When we say that a Language is closed it is always with respect to certain operation.
A simple example may be that the set of integers is closed under addition. It means when we take two numbers from set of integers say 3, 7 the result of their addition would also be in the set of integers.
Similarly if the result of an operation on the words of a language results in the word of the same language we say that the language is closed under that operation.