This may not be perfect grammar but I think what they are saying is that you know something so solidly that it is "in there". It is not just an intellectual learning, it is something you know deep within. So what does this imply for the person that knows in their heart? It implies that they really do know. I am not sure what else we can say here, but that they really do know. Now I will add that when a person really does know, they have more of an unshakable quality in their life. They can't be dissuaded from what they know. I know that it is now day-time, and that is an obvious one. Anyone who tries to talk me out of the fact that it is day-time, no, I am unshakable because I absolutely know it is day-time.
Now, in Buddhism, "someone who knows" is a person who has reached the first level of enlightenment called a Stream-Enterer. They know that what the Buddha taught is True. One of the qualities that comes for a person has reached the first-stage of enlightenment is that they have unshakable confidence in the Buddha, Dhamma, and Sangha. No matter what anyone says to them, their confidence will never, ever change. And when you actually know something in your heart absolutely, then there is an unshakable quality in you.