What does Mark 13:15 mean?

Jesus is prophesying about the events during the second half of the tribulation. When the Antichrist erects a statue in the temple and demands to be worshiped as a god, the Jews will have almost no time to flee their homes to hide in the mountains.

This warning was partially fulfilled during the destruction of Jerusalem in AD 70 when the Roman army destroyed Jerusalem and the Jews fled in the great Diaspora. Houses in Israel typically included a flat roof covered in rushes where those in the house would pray, socialize, or even sleep. It was on such a roof that Peter had a vision through which he learned Jesus had sanctified all food to eat and that the gospel was to be preached to the Gentiles, too (Acts 10:9–48). The rooftops were accessible through an exterior stairway. The suddenness described here implies Jews will not even have time to race down the stairs and duck into their homes to gather food or a staff.

But the prophecy can't mean just the AD 70 event. Jesus establishes that at this moment He does not know when He will return: He does not know the timing of the end of the tribulation (Mark 13:32). It follows that He does not know the mid-point of the tribulation, when the Jews will have to flee. This makes Him concerned that it will happen in winter, when the rainy season will swell the streams and the mountains may see snow (Mark 13:18).

Unlike the second coming, there's no indication that the timing of the destruction of the temple is hidden from Jesus. In fact, the end of the siege and the fall of Jerusalem occurred in late August to early September. Jesus' prophecy has hints of AD 70, but it will not be completely fulfilled until the tribulation. At that time, the inhabitants of Jerusalem and across Judea won't have time to gather money, IDs, or passports. They will be lucky to escape with their lives.