What does Revelation 16:2 mean?

When the first angel emptied his bowl onto the earth, grievous sores broke out on those who had the mark of the beast on the forehead or right hand (Revelation 13:16–18). These were unbelievers who worshiped the beast. The sores would certainly produce marks all over their bodies, just as they bore the mark of the beast. They had blasphemed God by practicing idolatry and would now pay the price. The Greek term here is helkos, which can refer to an ulcer or abscess—this is not a minor rash.

Centuries earlier the Philistines attempted to corrupt the Israelites' faith and faced stern judgment at God's heavy hand. He struck them with tumors (1 Samuel 5:6). When God struck Egypt with plagues, He caused boils to break out on the Egyptians' bodies (Exodus 9:8–11). Undoubtedly all business and social life will come to a grinding halt because of the severe pain humans experience from the sores. Because this bowl judgment targets the kingdom of the beast, we can conclude that the bowl judgments occur in the second half of the tribulation.

These same sores are mentioned during future plagues (Revelation 16:11), so these bowl judgments are not entirely separate from each other. At the very least, the effects of this judgment will not have ended before the next ones begin.