Natural language is the way that people talk and write to each other. It's the words and grammar we use to communicate with each other.
Imagine if you were trying to have a conversation with a friend, but instead of talking in a way that he or she could understand, you were speaking in a language they had never heard before. They wouldn't be able to understand you and you wouldn't be able to communicate with each other.
The same thing happens when computers or other machines try to understand the way we talk or write. They are programmed to understand certain rules and patterns in our language, but it's not always easy for them to understand all the subtleties and variations in the way we use words and grammar.
Natural language is important because it's how we communicate with each other every day. If computers and machines can better understand natural language, they can help us with many tasks, like searching for information online, answering questions, or even writing documents for us.