The Pentateuch is the first five books of the Old Testament or the Hebrew Bible (based on perspective and religious tradition). These books are arguably some of the most important books.
As for what is fundamental, I suppose the best way to proceed is to ask what elements are so central in these books that if we took them out, these points would change the essential character of the Pentateuch. Three points come to mind.
First, we need to mention creation. According to the Pentateuch, God created the heavens and the earth. Moreover, he created man and woman. This means that God is the creator, and as such he has a special relationship with the world and humanity. We can also emphasize the importance of the Garden of Eden, the original paradise to which the rest of the Pentateuch leads, namely the Promised Land, the land flowing with milk and honey.
Second, we need to mention the importance of Abraham. He is the father par excellence of Israel. He is the one with whom God made his first covenant. In short, God promises to bless him, so that he would be a blessing to others. No wonder people look at him as father Abraham, a man of great faith and favor. God says:
“I will make you into a great nation, and I will bless you; I will make your name great, and you will be a blessing. I will bless those who bless you, and whoever curses you I will curse; and all peoples on earth will be blessed through you.”
Third, there is Moses. Moses is the greatest leader, arguably, in the whole Bible. He leads the Israelites out of slavery from Egypt. He also leads the Israelites in the wilderness towards the Promised Land. He is also a great intercessor, judge, and lawgiver. For example, it is under him that the Ten Commandments are given.
By taking out any of these three elements, the Pentateuch would no longer be the same collection of books.