Each opening of a seal is followed by some event or series of events.
When each of the first four seals is opened, a horse and its rider appear and are described. These are commonly referred to as the four horsemen / four horses of the Apocalypse.
The opening of the fifth seal is followed by a vision of those that were "slain for the word of God". (Revelation 6:9)
When the sixth seal is opened, there is a "great earthquake," and signs appear in heaven. (Revelation 6:12-6:14) Also, 144,000 servants of God are "sealed ... in their foreheads" in Revelation 7.
When the seventh seal is opened, seven angels with trumpets begin to sound, one by one. The events of the seventh seal are further subdivided by the events following each angel sounding their trumpet. This seal is opened in Revelation 8, and the seventh angel does not sound until Revelation 11.
First Seal - Conquest, White horse
Second Seal - War, Red horse
Third Seal - Famine, Black Horse
Fourth Seal - Death, green or pale horse
Fifth Seal - Vision of martyrs
Sixth Seal - Earthquake and the marking of the 144,000
Seventh Seal - Trumpets of Angels and the end of the world
Bible scholars associate the seven seals with the seven Spirits of God, and other Biblical 'sevens'. The seals contain symbols commonly interpreted as death, famine, world wars, martyrdom, earthquakes, and the Antichrist. It also states that there will be "seven trumpets" announcing aspects of the "End Times": mankind being judged, seas turning to blood, sores on people's bodies, plagues, infertility, and the introduction of "seven bowls" (in King James Version called "vials"). These bowls are a third each of the sea, humankind, water, animal life, ships, crops, and earth, all engulfed by an infinite abyss.