What is the Balfour Declaration?

Just before Israel became a state, it was ruled by the British Empire. On November 2, 1917, Lord Balfour of Great Britain wrote a letter promising a “Jewish home” to the Jewish people in the land of Palestine. This declaration was later ratified by the League of Nations in 1922 and was the guiding document for the administration of the land until the UN voted for partition in 47 and the British mandate was no longer. Why is this document so important? What are the ambiguities in the text? What is the backstory and how did it come to be? Why was it so controversial?