This section is often called the "little apocalypse" in Mark. It is written in a different style than the rest of the gospel and utilizes much more symbolic language (dark suns, powers shaken, etc) than the rest. The difficulty for most modern readers is that the chapter is really about two different things and we often merge them or misunderstand them. The two issues are:
1. Jesus is predicting the impending destruction of the temple. History records a Jewish revolt that began in the late AD 60's (66-67). This revolt "liberated" Jerusalem and two other fortresses moving south and east from Jerusalem (the Herodian and Masada). In response to this circumstance, Imperial Rome sent General Titus and the Roman army who laid siege to Jerusalem, eventually captured the city and completely destroyed it. Titus reports to his superiors that he burned the buildings, broke down the walls, cut down the trees, salted the fields and "did not leave one stone standing upon another". By the way, the famous Western or "Wailing" wall in Jerusalem is the foundational retaining wall of the temple mount. When Jesus speaks of the Persecution, being on trial, etc. he is speaking of the days leading up to this destruction. The temple was destroyed in AD 70 and has never been rebuilt. The current Muslim holy site (The Dome of the Rock) currently occupies much of the region where the Temple originally stood. It is worth noting that Jesus words are spoken 40 years before the event and the writing of Mark's gospel was, at best guess, 15 years before Jerusalem's destruction.
2. The second piece begins at v 24 and is speaking of the second coming of Jesus. The critical understandings in this section are that A) Jesus is coming back; and B) no one knows when (see verse 32). When people attempt to stir others up with predictions of Jesus return or other doom and gloom end of the world predictions -- Biblically they will never work. Cannot know when Jesus is coming back all we can truthfully know is that he IS coming back and that we should keep awake and a watchful eye.