When you see a truly great performance, there will be no visible signs of memorization. And yet, they first memorized their messages, and then went way beyond that; so that what you see, is someone speaking from the heart.
That is what it means to know something by heart.
Memorizing the words is only half the job and sorrowfully insufficient.
The audience didn’t show up to hear your words. They came to hear what you had to say from the heart.