What does Daniel 9:20-27 mean?

While I was speaking and praying, confessing my sin and the sin of my people Israel, and presenting my plea before the LORD my God for the holy hill of my God, while I was speaking in prayer, the man Gabriel, whom I had seen in the vision at the first, came to me in swift flight at the time of the evening sacrifice. He made me understand, speaking with me and saying, “O Daniel, I have now come out to give you insight and understanding. At the beginning of your pleas for mercy a word went out, and I have come to tell it to you, for you are greatly loved. Therefore consider the word and understand the vision. “Seventy weeks are decreed about your people and your holy city, to finish the transgression, to put an end to sin, and to atone for iniquity, to bring in everlasting righteousness, to seal both vision and prophet, and to anoint a most holy place. Know therefore and understand that from the going out of the word to restore and build Jerusalem to the coming of an anointed one, a prince, there shall be seven weeks. And for sixty-two weeks it shall be built again with squares and moat, but in a troubled time. And after the sixty-two weeks, an anointed one shall be cut off and shall have nothing. And the people of the prince who is to come shall destroy the city and the sanctuary. Its end shall come with a flood, and to the end there shall be war. Desolations are decreed. And he shall make a strong covenant with many for one week, and for half of the week he shall put an end to sacrifice and offering. And on the wing of abominations shall come one who makes desolate, until the decreed end is poured out on the desolator.”

English Standard Version (ESV)