Rev. 6:1 Now I watched when the Lamb opened one of the seven seals, and I heard one of the four living creatures say with a voice like thunder, “Come!” 2 And I looked, and behold, a white horse! And its rider had a bow, and a crown was given to him, and he came out conquering, and to conquer. 

Rev.6:3 When he opened the second seal, I heard the second living creature say, “Come!” 4 And out came another horse, bright red. Its rider was permitted to take peace from the earth, so that people should slay one another, and he was given a great sword.

Rev 6:5  When he opened the third seal, I heard the third living creature say, “Come!” And I looked, and behold, a black horse! And its rider had a pair of scales in his hand. 6 And I heard what seemed to be a voice in the midst of the four living creatures, saying, “A quart of wheat for a denarius, and three quarts of barley for a denarius, and do not harm the oil and wine!”

Rev 6:7 When he opened the fourth seal, I heard the voice of the fourth living creature say, “Come!” 8 And I looked, and behold, a pale horse! And its rider’s name was Death, and Hades followed him. And they were given authority over a fourth of the earth, to kill with sword and with famine and with pestilence and by wild beasts of the earth.

These four horsemen represent war, famine, and death.
Who or what are these horsemen?
The images in Revelation are often images from the Old Testament. In the Old Testament, these images are often explained by angels. The same is true of the horsemen of the Apocalypse.
We read in the Bible, in the book of Zechariah, chapter 6:1-3

1 Again I lifted my eyes and saw, and behold, four chariots came out from between two mountains. And the mountains were mountains of bronze. 2 The first chariot had red horses, the second black horses, 3 the third white horses, and the fourth chariot dappled horses—all of them strong.

Secondly, they stand before the Lord of the earth.
In the biblical book of Daniel, we find the number of angels who stand before the Lord of the earth. We read in Daniel chapter 7:9-10

Dan 7:9 “As I looked, thrones were placed, and the Ancient of Days took his seat; his clothing was white as snow, and the hair of his head like pure wool; his throne was fiery flames; its wheels were burning fire.10 A stream of fire issued and came out from before him; a thousand thousands served him, and ten thousand times ten thousand stood before him; the court sat in judgment, and the books were opened.

We also find the number of angels in Revelation. We read Revelation chapter 9:15-16

Rev. 9:15 So the four angels, who had been prepared for the hour, the day, the month, and the year, were released to kill a third of mankind. 16 The number of mounted troops was twice ten thousand times ten thousand; I heard their number.

Rev7:1 After this I saw four angels standing at the four corners of the earth, holding back the four winds of the earth, that no wind might blow on earth or sea or against any tree.

We have almost the same picture as in Revelation chapter six, namely a white, red, black, and spotted horse, which is equivalent to a pale horse. Who these horses are is explained by an angel in Zechariah chapter 6:4-5.

4 Then I answered and said to the angel who talked with me, “What are these, my lord?” 5 And the angel answered and said to me, “These are going out to the four winds of heaven, after presenting themselves before the Lord of all the earth.

We have two clues here.
1. They are winds.
We find the winds again in Revelation chapter 7:1.

The Four Horsemen of the Apocalypse are two hundred million angels who are held captive in Revelation chapter seven and released in chapter nine.

The Four Horsemen of the Apocalypse