The Six Seals and the 144,000
Remember that in yesterday's reading there was a scroll that was sealed with seven seals. The only one worthy to open those seals is Jesus, and in today's passage He begins to break the seals, bringing forth events with each broken seal. The appearance of Jesus to break open the seals indicates that the end of all things are at hand and the last days have truly begun - the Tribulation is at hand.
Again, not everyone agrees on the symbolism here, so I'm going to tell you the view that seems to have the most support:
First seal: Rider on the white horse - This is most likely the Antichrist, coming to rule over the nations of the earth in false reign. See Daniel 9:26 for some context - this is often cross-referenced here.
Second seal: Rider on the red horse - This initiates a time of war and bloodshed on the earth that will ultimately climax with the battle of Armageddon.
Third seal: Rider on the black horse - This speaks of famine conditions that overtake the world due to the season of wars initiated by the previous rider.
Fourth seal: Rider on the pale horse - A killing of one fourth of the world's population.
"Because of the current world population being so great, one fourth of the inhabitants would represent a number larger than those destroyed in Noah’s day. Therefore, this would be the “Great Tribulation” of unprecedented magnitude spoken of by Jesus in Matthew 24:21. Since “Great Tribulation” is the term that technically refers to only the final three and a half years of the seven-year Tribulation, some (e.g., Walvoord) believe that these seals apply strictly to that end period." 
Fifth seal: Martyrs - These could possibly be those killed for their faith throughout the ages, but more likely those who die for their faith during the Tribulation, since that is where this seal is placed.
Sixth seal: Cosmic disturbances -
"Ryrie (in company with Walvoord, Morris, and others) takes this passage quite literally (except in the case of stars falling, which he thinks refer to a meteor shower). He identifies six catastrophic events calculated “to strike terror into the hearts of men living on the earth. … At this point men will know assuredly that the tribulation has begun, for they recognize it as ‘the great day of his wrath.’ ” 
Which brings us to an interlude before the seventh seal. But what's going on with the 144,000? The seals have been pretty straightforward other than the horse symbolism, but what is this?
"During the Great Tribulation, a godly remnant of 144,000 Jewish people will be sealed (v. 4) for protection from the later plagues (cf. 9:4)...That these are physical Israelites (not to be confused with the church, as in the historicist and spiritual approaches) is underscored by the division of the group into twelve tribes. The church is not divided into tribal divisions." 
So - during this time there will be a turning of a significant number from Israel to God as they recognize God's hand in what is happening. See Romans 11:25 as a possible reference to this.
I hope that helps! Questions?
 Ibid, Re 6:12–17.
 Ibid, Re 7:4–8.