Essence
Soul, True Meaning
noun, の adjective
A god's bone marrow contains their essence and soul. Don't go messing around with the bone marrow of gods trying to confirm this, though — you'll probably end up getting smote.
神髄 is just another way of writing 真髄, so it shares its nuance of describing the deepest meaning or most important part of something. 真髄 actually started out as an incorrect spelling of 神髄, but got promoted to a legit alternative after enough people started using it.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
プロの神髄を見せつけられました。
I was shown what it truly means to be a professional.
この考え方こそが、日本文化の神髄です。
This way of thinking is the very essence of Japanese culture.
彼は武道の神髄をよく理解している。
He understands the true essence of martial arts well.
神髄を知らずに、形だけ真似ても意味がない。
There's no point in copying the form without knowing the essence.
この作品の神髄は、人間の弱さを描いた点にある。
The essence of this work lies in its portrayal of human weakness.