“If language is not correct, then what is said is not what is meant; if what is said is not what is meant, then what must be done remains undone; ...Hence there must be no arbitrariness in what is said. This matters above everything.”
This means that when you make a mistake, you say something different. If you say something different, what should happen never happens. So you cannot say anything you want. You need to be careful, and say what you mean. Mistakes can make a big difference. This is the most important thing.
Think about these two sentences:
A small change can make a big difference. In this case, it is the difference between closing a window and turning on a heater (1) and taking him to a doctor to get some medicine (2). Languages are full of small words that make BIG differences. Can you think of some?
It's OK to make mistakes when you are learning. We all do. But it doesn't mean that you should not care about mistakes. Languages have rules and patterns. If you don't use them, you can say something strange, or crazy, or mysterious. Making mistakes is how we learn, but only if you know it is a mistake.
You must be careful. You must pay attention. You must care about your mistakes if you want to be better at English. Just never let fear of mistakes be a problem.