What does it mean to boast against the branches?

But if some of the branches were broken off, and you — a wild olive — were grafted in among them and have become equal sharers in the rich root of the olive tree, then don't boast as if you were better than the branches! However, if you do boast, remember that you are not supporting the root, the root is supporting you. So you will say Branches were broken off so that I might be grafted in. True, but so what? They were broken off because of their lack of trust. However, you keep your place only because of your trust. So don't be arrogant; on the contrary, be terrified! For if God did not spare the natural branches, he certainly won't spare you! So take a good look at God's kindness and his severity: on the one hand, severity toward those who fell off; but, on the other hand, God's kindness toward you — provided you maintain yourself in that kindness! Otherwise, you too will be cut off! Moreover, the others, if they do not persist in their lack of trust, will be grafted in; because God is able to graft them back in. For if you were cut out of what is by nature a wild olive tree and grafted, contrary to nature, into a cultivated olive tree, how much more will these natural branches be grafted back into their own olive tree! (Romans 11:17-24 CJB) But if you call yourself a Jew and rest on Torah and boast about God and know his will and give your approval to what is right, because you have been instructed from the Torah; and if you have persuaded yourself that you are a guide to the blind, a light in the darkness, an instructor for the spiritually unaware and a teacher of children, since in the Torah you have the embodiment of knowledge and truth. (Romans 2:17-20 CJB)