Revelation 7:1-8 – God’s People Are Marked with a Seal.

Revelation says that the redeemed bear a seal on their foreheads. The seal consists of the names of God and the Lamb and shows that the redeemed belong to God (Revelation 14:1). The seal is presumably not a visible one but a way of indicating that people find their identity in relationship to God and Christ. This seal or relationship does not mean that people are exempt from all earthly suffering, but it does show that God has claimed them and gives them the promise of life everlasting. John hears that those who are sealed number 144,000 from the 12 tribes of Israel (7:4), but when he turns to look at them he discovers that they are actually a countless multitude from every tribe and nation (7:9). The 144,000 are not a special ethnic group. The imagery describes the whole people of God. The seal that they bear is the opposite of the mark of the beast, which is introduced in Revelation 13:11-18.

https://enterthebible.org/passage/revelation-71-8-gods-people-are-marked-with-a-seal

Revelation 7:3–4 and 9:4 refer to group of people who have the seal of God, and thus His protection, during the tribulation. During the fifth trumpet judgment, locusts from the Abyss attack the people of the earth with “power like that of scorpions” (Revelation 9:3). However, these demonic locusts are limited in what they can harm: “They were told not to harm the grass of the earth or any plant or tree, but only those people who did not have the seal of God on their foreheads” (Revelation 9:4). The individuals who are marked by God are preserved. The seal of God during the tribulation is the direct opposite of the mark of the beast, which identifies people as followers of Satan (Revelation 13:16–18).

https://gotquestions.org/seal-of-God.html