But if some of the branches were broken off, and you, although a wild olive shoot, were grafted in among the others and now share in the nourishing root of the olive tree,  do not be arrogant toward the branches. If you are, remember it is not you who support the root, but the root that supports you. (ESV)
I am not an expert when it comes to things like olive trees. There are some things that we need to understand about olive trees if we are to understand what Paul is saying here in these verses. Apparently, olive trees can live for hundreds of years. Branches will sometimes die and no longer offer any fruit. Wild branches are then grafted in and are able to connect with the root and live.
Paul uses this wonderful picture of olive trees that his original audience would have been very familiar with. He talks about some of the branches being broken off. These are the Israelites who had fallen away in their unbelief and rebellion. The wild olive shoots that were grafted in are the Gentiles who have now responded to the Gospel of Jesus Christ with joy.
In verse 18 Paul is giving warning to the Gentiles. They would have seen the way in which the Jews had persecuted the Apostle Paul and ultimately the Church of Jesus Christ. It would have been easy for them to look down their collective noses at the Jews. They could have been arrogant, thinking that they were in some way superior to the people of Israel who had been broken off like branches.
Paul reminds us all of where the root is. The root is the covenant that was started with Abraham himself. The people of Israel were part of the root. We are also part of the same root. We are in no condition to think that we are better than anyone. If it were not for the grace and calling of our wonderful God we would be just as dead as broken off branches.
As you look at the people of Israel your heart should be filled with love and compassion. We have the same root together. Be a thankful branch. Allow the love of God to fill your heart as he bears his fruit in you, his branch.