This proposal was made by Theodore Herzl in 1903, reluctantly, toward the end of his life when he found that all his efforts had made absolutely no progress toward convincing the British (occupying Palestine) that the Jews deserved a homeland in Palestine. His attitude was that, as ridiculous as a Jewish homeland outside of Palestine might seem, it could still function to save Jewish lives in the event that any Jewish community anywhere in the world faced fresh persecutions. (It was the roaring anti-Semitism that erupted in France during the Dreyfus affair that convinced Herzl the Jews must have a homeland.) He wondered if the British might be willing to give Jews a place of their own outside of Palestine, and at various times considered Madagascar, and Uganda.
The idea got virtually no support from any of the other Zionists. The reason why is interesting to consider. Herzl was rather unique, in that he was utterly assimilated, having grown up with nearly no knowledge of the religion, or Jewish values and principles. (It was probably this external perspective that enabled him to see the Jews as a national group, instead of as a religious group.) But, for the other Zionists, who spanned a wide range of religious practice yet had all grown up with a better understanding of Jewish ideas, it was inconceivable that any land other than the ancient homeland be considered for the location of a Jewish state. So, in short, the Uganda proposal very quickly went nowhere. Today, it is hardly even recalled except by those who are making polemical arguments against Zionism.