Abu Talib was the leader of the Banu Hashim clan and the uncle of the Prophet ﷺ. When the Prophet ﷺ began preaching openly, some of his fellow clansmen went to Abu Talib. They complained that his nephew was cursing their idols and preaching against the ways of their forefathers. In fact the Prophet ﷺ never cursed their idols, since the Qur'an forbids it. Abu Talib wanted to avoid a confrontation, so he gave them some gentle words and let the matter go.
But the matter did not go away. More people converted, and visitors to Makkah carried the news of the new message home with them. A few weeks later the men came back and put more pressure on Abu Talib. They demanded that he either stop his nephew from preaching or hand him over to them to do with as they pleased. By Arab custom, no one could harm a member of a tribe unless its chief allowed it. So as long as Abu Talib protected him, they could do nothing.
Abu Talib went to his nephew and begged him to have mercy on himself and on his old uncle. In what is probably the more authentic version of his reply, the Prophet ﷺ asked him, "Do you see the sun, O my uncle?" When Abu Talib said yes, the Prophet ﷺ said, "Wallahi, I have no more power to stop preaching than you do to light your stick with this sun." Seeing his sincerity, Abu Talib told him to do as he wished, and promised him his protection.