Here "the branches" is a metaphor that stands for the Jewish people. Alternate translation: "do not say you are better than the Jewish people God has rejected" (See: [[rc://en/ta/man/jit/figs-metaphor]])
Again Paul implies that the Gentile believers are branches. God saves them only because of the covenant promises that he made to the Jews. (See: [[rc://en/ta/man/jit/figs-metaphor]])