Albert Barnes Commentary Matthew 24:29

"But immediately after the tribulation of those days the sun shall be darkened, and the moon shall not give her light, and the stars shall fall from heaven, and the powers of the heavens shall be shaken:" — Matthew 24:29 (ASV)

Shall the sun be darkened, etc. The images used here are not to be taken literally. They are often employed by the sacred writers to denote any great calamities. Just as the darkening of the sun and moon and the falling of the stars would be an inexpressible calamity, so any great catastrophe—any overturning of kingdoms or cities, or dethroning of kings and princes—is represented by the darkening of the sun and moon, and by some terrible convulsion in the elements.

Thus, the destruction of Babylon is foretold in similar terms (Isaiah 13:10), and that of Tyre (Isaiah 24:23). The slaughter in Bozrah and Idumea is predicted in the same language (Isaiah 34:4). See also Isaiah 50:3, Isaiah 60:19–20, Ezekiel 32:7, and Joel 3:15.

To the description in Matthew, Luke adds (Luke 21:25) that there would be distress of nations, with perplexity; the sea and the waves roaring; men's hearts failing them for fear, and for looking after those things which are coming on the earth. All these are figures of great and terrible calamity. The roaring of the waves of the sea denotes great tumult and affliction among the people. Perplexity means doubt, anxiety; not knowing what to do to escape. Men's hearts failing them for fear, or by reason of fear. Their fears would be so great as to take away their courage and strength.

Regarding the heavens shall be shaken, refer also to 2 Peter 3:10.