Although some interpretations differ, in most accounts, the four riders are seen as symbolizing Conquest, War, Famine, and Death, respectively. The Christian apocalyptic vision is that the four horsemen are to set a divine apocalypse upon the world as harbingers of the Last Judgment.
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 and see!" 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)
When the Lamb opened the second seal, I heard the second living creature say, "Come and see!" Then another horse came out, a fiery red one. Its rider was given power to take peace from the earth and to make men slay each other. To him was given a large sword. (Revelation 6:3-4)
When the Lamb opened the third seal, I heard the third living creature say, "Come and see!" I looked, and there before me was a black horse! Its rider was holding a pair of scales in his hand. Then I heard what sounded like a voice among the four living creatures, saying, "A quart of wheat for a day's wages, and three quarts of barley for a day's wages, and do not damage the oil and the wine!" (Revelation 6:5-6)
When the Lamb opened the fourth seal, I heard the voice of the fourth living creature say, "Come and see!" I looked and there before me was a pale horse! Its rider was named Death, and Hades was following close behind him. They were given power over a fourth of the earth to kill by sword, famine, and plague, and by the wild beasts of the earth. (Revelation 6:7-8)