Looking at all the books and articles that have been written on this chapter one can quickly discern that it has been understood in many differing ways. Some see it primarily or completely looking to the 2nd coming of Jesus. Many others see it as mixing together the 2nd coming of Jesus with His coming in judgment upon Jerusalem in A.D. 70. Yet others view it as completely discussing the destruction of Jerusalem with no reference to the end of the universe at all.
For most modern readers, the problem understanding comes from being unfamiliar with the nature of apocalyptic writings. In such writings events of this age are described in cataclysmic language. Wars and destruction of worldly kings, empires, and cities are described as the stars falling from heaven, the Sun becoming dark, etc. Thus, when modern readers read this chapter they can only visualize the end of the world.
My take on this chapter is that it discusses the destruction of Jerusalem up through verse 34. There Jesus concluded his discourse on the destruction of the temple and Jerusalem saying: “Truly, I say to you, this generation will not pass away until all these things take place.” I understand Jesus to be using the same cataclysmic language as the prophets when they discussed the destruction of various empires and cities. I would suggest that you get a bible with good cross-references (or a book like Treasury of Scripture Knowledge which consists entirely of a book-length listing of cross-references) and become familiar with the language of the prophets. [Check Isa. 13:10; Ezek. 32:7; Joel 2:10, 31; Acts 2:20; Joel 3:15; Amos 5:20; 8:9; Zeph. 1:15 for starters of your study]
With verse 35, “Heaven and earth will pass away, but my words will not pass away”, Jesus discusses the fact that the very end is of an unknowable day and time. Not knowing WHEN should cause one to be constantly prepared. Thus, we see a transition into what some have called ‘the parables of preparedness’. In these parables Jesus exhorts disciples in this very thing, that we should be prepared at all times and in all ways.
The next chapter ends with the great scene of final judgment and eternal goodness in the presence of God or eternal misery in the place that was prepared for Satan and his angels. The question of each moment of each day is “Am I ready?” Are you?