We find ourselves looking at the start of a mini interlude at the beginning of chapter 7. Sitting between seals 5 and 6 and the last seal, 7. Chapter 7:1-8 explains how believers are protected from the first 4 tribulations. The opening words "after this" reference, not a sequence or a consecutive point, but rather a break-in time. We find 4 angels standing at the 4 corners holding back the 4 winds of the earth. This grouping of 4s easily points back to the first 4 seals where we met the 4 horsemen, Conquering, War, Famine, and Death. We can now resolve that these plagues are either during the time of Rome (the great empire) or at least before the Great Tribulation. This scene draws from Isaiah 11:12 with the 4 corners representing the whole earth. The whole earth that is under the sovereignty of God. We then see and hear a 5th angel who tells the other 4 not to harm the land or the sea or the trees until a seal (or mark) has been put on the foreheads of the servants of our God. Seals 1-6 harm the earth and its inhabitants, however, the servants of God will be protected in some way.
What does the seal on the forehead do? Will it protect us from physical harm, or from demons? We know, don't we, that believers and non-believers experience similar physical issues and we know that trials purify God's servants. So no, rather it is highly likely that the mark will protect us from losing our faith (and therefore salvation). See Ezekiel 9 which also speaks of a mark on foreheads.
Interestingly the word "mark" used in Ezekiel 9 is "tav". Tav (also spelled Taw) is the twenty-second and final letter of the Hebrew alphabet. Its meaning is truth, sign, but also life or death. It is often represented as a cross symbol: "x" or "+". More info on this word can be found here.
Now we see who is sealed, or marked. Who has this membership and who will enter into the eternal city of God... the number of those who were sealed: 144,000 from all the tribes of Israel. Does this mean only 144,000? No, this number is likely only to be a symbolic reference to the New Israel. 1000 men make up a battalion, 12 battalions from each of the 12 tribes makes 144,000. This is a representation of the redeemed church. Not a defined or definitive number. We know this because all redeemed believers are included when the word "servants" is used elsewhere in Revelations. Ezekiel 9 which is the Old Testament reference being used in this new context doesn't have a restriction referenced and finally, if the beast's seal (chapter 13) is on all of his people, wouldn't God do the same? We are therefore not talking about a special group of martyrs, but rather linking the church back to the Jewish heritage, to which the Gentiles have been folded into through faith in Jesus.
We are left with this knowledge, we are God's army of redeemed souls, and that we have nothing to fear because we have been sealed by God.
