This is the 91st most frequent Russian word.
не
"Не" means "not" or "no" (negation).
The word "не" negates the verb "пришёл" to form the negative statement about his action.
Он не пришёл вчера.
He didn't come yesterday.
In this case, "не" negates the ownership implied by "моя книга" indicating it's someone else's book.
Это не моя книга.
This is not my book.
Here, "не" is used with the verb "знаю" to express the lack of knowledge.