"'When you see "the abomination that causes desolation" standing where it does not belong - let the reader understand - then let those who are in Judea flee to the mountains.'" NIV translation
This period of time that is referred to as the "great tribulation" begins with "the abomination that causes desolation" which is a picture of the anti-christ. Jesus was talking about the fact that an idol will be set up in Jerusalem just as it was in the time of Antiochus Epiphanes. Daniel referred to this in his visions of the end times (see Daniel 9). It is also what John was talking about with the people worshiping the beast in Revelation 13.
"Flee to the mountains" refers to the Transjordan Mountains. During the reign of Antiochus Epiphanes, many of the faithful Jews fled to the city of Pella which is located in the modern country of Jordan. In the last days, God's people will once again flee from the anti-christ.
"'Let no one on the housetop go down or enter the house to take anything out. Let no one in the field go back to get their cloak. How dreadful it will be in those days for pregnant women and nursing mothers! Pray that this will not take place in winter, because those will be days of distress unequaled from the beginning, when God created the world, until now - and never to be equaled again.'" NIV translation
This will happen so fast that people will not have any time to gather their things. The times will be so terrible for those who are not prepared to move quickly such as mothers with small children. Times are rough now but, even the worst of days that we have seen are not as bad as these times.
"'If the Lord had not cut short those days, no one would survive. But for the sake of the elect, whom he has chosen, he has shortened them.'" NIV translation
These days are numbered as God allows Satan to exercise some control over the events on the earth. God limited the number of these days for the good of His people. There will be a remnant of His people that are left as a witness on the earth and, if He had not shortened the days, even they would perish.