I watched as the Lamb opened the first of the seven seals. Then I heard one of the four living creatures say in a voice like thunder, “Come!” 2 I looked, and there before me was a white horse! Its rider held a bow, and he was given a crown, and he rode out as a conqueror bent on conquest. (Revelation 6:1-2)
After the Lamb (Jesus) opens the first of the seven seals, the first thing that John sees is the white horse and rider described in verse 2. The crown on his head indicates that this rider is a ruler, and the verse clearly explains that He is a great conqueror.
At first one might be tempted to conclude that this person is Jesus. After all, Jesus is also pictured as a great ruler and conqueror later on in Revelation:
I saw heaven standing open and there before me was a white horse, whose rider is called Faithful and True. With justice he judges and wages war. 12 His eyes are like blazing fire, and on his head are many crowns. He has a name written on him that no one knows but he himself. 13 He is dressed in a robe dipped in blood, and his name is the Word of God. (Revelation 19:11)
But this rider described in Revelation six is not the same person. While Jesus came to bring peace by instantaneously destroying the armies of evil, the rider in chapter 6 brings constant warfare that never results in peace, but just more war. This rider is probably the antichrist. It fits descriptions of him that we see elsewhere in the Bible including numerous descriptions in the book of Daniel. The fact that he is dressed in white probably indicates that he is a counterfeit Messiah, attempted to appear as Jesus. However, true believers who are on Earth at this time need not be fooled by the antichrist when he comes, for Jesus Himself will not come to Earth until after the tribulation is over and the antichrist is defeated (Matthew 24:29-31, Revelation 19, II Thessalonians 2.