Not all languages are written. Language is, first and foremost, oral: we speak and hear a language. Writing a language means we are giving it a visual representation, that is, a graphic representation where we use graphic symbols to represent and transcribe the oral language visually. Like the oral language, the written language facilitates communication.
Why is writing important?
Spoken words are fleeting. Writing allows us to represent spoken language visually and therefore to conserve it for longer.