The olive tree is often used in the Bible to represent Israel. Jesus is the root that holds up and supplies nutrition and water to the tree. The branches are God’s people.
Throughout the history of Israel, some people believed in God and others rejected God. When they were rejected God cut them off the tree (20). When Jesus came most rejected him. God then spread the good news to the Gentiles. When the Gentiles believe in Jesus, God grafts them into the tree. So now they receive nutrition from the fruit Jesus just like Israel (17).
When anyone is cut off they can easily be grafted back onto the tree (23). There is always hope. All it takes is faith (20).