{It is like} some of the branches from an olive tree have been broken off, and the branch of a wild olive tree has been joined to that first tree. You non-Jews are the same as that wild branch, and you now share the strength and life of the first tree (the Jews).
So don't boast about those branches that were broken off. You have no reason to boast. Why? You don't give life to the root. The root gives life to you.
That is true. But those branches were broken off because they did not believe. And you continue to be part of the tree only because you believe. Don't be proud, but be afraid.
So you see that God is kind, but he can also be very strict. God punishes those people that stop following him. But God is kind to you, if you continue following in his kindness. If you don't continue following him, you will be cut off {from the tree}.
It is not natural for a wild branch to become part of a good tree. But you non-Jews are like a branch cut from a wild olive tree. And you were joined to a good olive tree. But those Jews are like a branch that grew from the good tree. So surely they can be joined to their own tree again.