What is meant by the Prophet's saying, "So inform us what has been and what is to be," and does it mean that Allah informed His Messenger of everything that will exist until the Day of Judgment, or only of the tribulations and matters of the End of Time, given that there is another hadith indicating his lack of knowledge of people's affairs after him?

The unseen is known only to Allah, except for whomever He, the Glorified, chooses from His messengers. Allah has informed His Prophet Muhammad, peace and blessings be upon him, of many future unseen matters as a miracle for him. However, the details of the unseen are encompassed by none but Allah. As for the hadith mentioned in the question, "Then he informed us of what has been and what is to be," it is an authentic hadith. This generality is not intended in its absolute sense, but rather it is from the general that is intended to be specific. This is because the details of what has been and what will be are known only to Allah. What is meant by it are the great matters, momentous events, signs of the Hour, and the tribulations that will befall the Ummah. The hadith of the Hawd (Pond) and what was said to the Prophet, peace and blessings be upon him: "Indeed, you do not know what they innovated after you," indicates that the Prophet, peace and blessings be upon him, did not know the details of the unseen, and that what is meant by the hadiths in which he informs of what is to be are the great and momentous matters.