What does Matthew 24:21 mean?

Jesus has been describing the need for those in Jerusalem to run for the hills—to flee immediately—when the sign of the abomination of the temple takes place (Matthew 24:15). There won't be time for grabbing supplies. Pregnant and nursing mothers will suffer enormously. He has told His disciples to pray that they will not need to run in winter or on the Sabbath (Matthew 24:16–20).

Here, Christ refers to the era marked by this act of violation as a "great tribulation." He says the horrors of this time will be the worst of all human history.

Those who believe Jesus' prophecy was fulfilled in AD 70 point to the Roman siege, genocide of the Jews, and demolishing of the temple. Many Jewish people did run for the hills, some successfully. Historians report that the community of Christians living in Jerusalem at the time did escape to Pella, which is east of the Jordan River and about 65 miles from Jerusalem. Historians describe the suffering of those days in Jerusalem as epic in its terror. The siege itself led to disease and famine, with tales of mothers eating their own children to survive. Once the walls were breached, the Romans were savage in their slaughter of the Jewish people as they laid waste to the great city.

However, in those events, the temple was violated after most of the other events had occurred. It did not happen first, allowing people to see it as a warning sign and then run. As violent as those days were, they did not threaten to wipe out all of mankind (Matthew 24:22).

More likely is that Jesus is referring to the last years (Matthew 24:14) prior to His second coming (Revelation 19:11–15).