the sign of the Son of Man: The sign is probably the Son of Man himself (see Matthew 12:38–42; 16:4). His coming shows that God has vindicated him (proved him right; see 26:64). It will also cause deep mourning for those who face God’s judgment.
all the tribes of the earth: This could also mean "all the tribes of the land." The Greek term used here (phulai) often refers to the tribes of Israel. It is not the usual term for Gentiles (ethnoi). This suggests that it may refer to a national disaster in Israel (see Matthew 21:43; Zechariah 12:10–14).
the Son of Man coming on the clouds of heaven: See Daniel 7:13. Many think this means the Son of Man will descend from God's presence to live on earth as judge and savior. This is the second coming of Christ. Others believe it describes the Son of Man coming in judgment, but not necessarily as the second coming of Christ (see Matthew 10:23; 16:27; 19:28; 25:31; 26:64; 28:18).