The rules determine such things as what is a word and what is not a word, as well as what new words are possible. We can break the rules and create cxf, lfc, and afx. We quickly recognize that these cannot be English words, even without being aware of the rule that lets us make this judgment. Here are more of the characteristics of a language. Languages have rules or structure.
Languages combine small units into more complex units.
Some units or combination have meaning. Others do not. Words are not a part of language unless they have meaning. Think about what you are doing right now. As I read I hear the sound of the words in my head.