Language is a code, a system that allows communication between its speakers. Communication requires not knowing the grammar rules or using phrases that are grammatically correct, but using them in the  appropriate comunicative  circumstances.

The purpose of the communicative approach is to gradually acquire the language through procedures that promote the use of the target language for communicative purposes rather than memorizing rules or the use of specific structures. 

Therefore knowing a language, means “I know how to use the language according to the specific communicative purpose”.