A natural language is one that has evolved over time. It does not involve deliberate planning and strategy. Latin, English, Spanish, and many other spoken languages are all languages that evolved naturally over time.
Natural languages are different from formal or constructed languages, which have a different origin and development path. For example, programming languages including C, Java, Python, and many more were created for a specific reason.
For a machine to be autonomous, a key tenet is to be able to communicate via one of the natural languages known to humans. In the wide world of Artificial Intelligence, one field deals with enabling machines to interact using these languages: Natural Language Processing (NLP).