Paul continues to talk to the Gentile Christians with a cautioning tone. He doesn't want them to reach the wrong conclusion about the truth by being wise in their own sight. He wants them to understand one of the central mysteries of God's relationship with Israel and humanity at large.
God has brought upon Israel a "partial hardening." In other words, most Israelites have been hardened to faith in Christ by God, keeping them from receiving God's righteousness. Why has God done this? As Paul has made clear, it was to make room for the non-Jewish people of the world to come into relationship with God through faith in Christ.
However, Paul now states outright that this is a temporary season. Israel's hardening will end when the "fullness" of the Gentiles has come to God through faith in Christ. Does this mean that God is waiting for all Gentiles to believe in Jesus? That's not likely, nor does it fit the words used here. The "fullness of the Gentiles" means "as many as will believe," or "the complete number." When that mysterious point has been reached, the Jewish people will no longer be hardened in their unbelief in Christ. This does not mean, necessarily, that every Israelite will come to faith in Christ, but perhaps that a representative proportion of living Israelites will do so. Nor does it mean that no Jewish person, prior to then, can come to faith—but most will not.
Romans 11:25–32 wraps up a long section about God's plans for His Israelite people. In short, He plans to remove the hardening of unbelief and renew His covenant with Israel. This will occur as many Jewish people come to faith in Christ at some future time. Israel remains deeply loved by God because of His irrevocable promises and calling for the patriarchs. In the end, God will show mercy to all Jews and Gentiles who come to Him through faith in Christ for the forgiveness of their sins.