Jesus is speaking of signs that will accompany his return at the end of time. The sun will be darkened, the moon will not give out its light, stars will fall from the sky, and the Son of Man will come in the clouds in power and glory. The language is highly symbolic, as is typical of biblical vocabulary used to refer to the end of times. In the minds of many at the time of Jesus, the sun, moon and stars were celestial forces that guided and shaped history. The Gospel tells us that at the coming of the Lord, these phenomena will be seen as having no power at all. In other words, history is not in the control of any earthly or celestial reality. That control rests solely in the hands of God, who has given over the unfolding of all human history and its fulfillment into the hands of his Son Jesus.
This is very important to keep in mind as we are striving to interpret not future signs but present ones. In our day events are unfolding around us that feel as if the sky were falling, that all is turning dark and that forces inimical to us are in control of things.