These verses cannot be properly understood unless we remember that the break between them and the previous chapter is artificial. These verses were part of that discourse, and part of the original chapter.
Thus, we remember that Jesus had been describing the time of the gathering of the house of Israel. Those who heard them clearly understood that they had not yet been gathered, and so the next most logical question was when that would happen. That is the question these verses answer: “I give unto you a sign, that ye may know the time when these things shall be about to take place.” Jesus does not give a date, but a sign. That would not be a problem for a people who had lived through the signs of his birth and death, and now clearly understood that the signs, when fulfilled, would mean that the event they foretold was also real, and imminent.
What was the sign of the beginning of the gathering? It was the things that Jesus told them “shall be made known unto the Gentiles.” The Book of Mormon is the sign.