John is seeing the vision of the time of trouble during the last days before God’s kingdom. The faithful believers are to be sealed with the seal of God so that they might survive the tribulation. John hears a voice describing the sealed as coming from the 12 tribes of Israel. But when he looks, he sees that they come from every nation (implying that the believers form “spiritual Israel” and that the literal tribes of Israel are not intended here). Chapter 7 finishes with a poetic picture of those who survive the tribulation serving continually in God’s temple.
In Revelation 8, the next stage of the judgments of God are unleashed. Trumpets were used to announce important events. Here, they are announcing a series of plagues — symbolising God using the forces of nature in judgment against those who have oppressed his people. That only a third is affected in each case, along with the symbolism of the trumpets, suggests that the purpose of these judgments will be to warn people, giving them time to repent before God’s full wrath is poured out.