Language learners often think they know something (e.g. a word, phrase, grammar rule) because they understand it or because they have studied it before. However, there is a huge difference between understanding something and really knowing it.
'Really knowing' means being able to use the word, phrase or grammar rule.
For example, I'm sure you understood the vocabulary in this lesson. But does that mean that you know it? Could you use those words and phrases in different contexts in your own essays or spoken answers?