The dead in Christ who rise first at the rapture are those believers who have died in the faith. They will be resurrected first to glorified bodies before those Christians who are alive at the time of the return of Christ. Let's take a closer look.
"For the Lord Himself will descend from heaven with a shout, with the voice of the archangel, and with the trumpet of God; and the dead in Christ shall rise first," (1 Thess. 4:16).
This is the only place in Scripture where it occurs. But what does "dead in Christ" mean? It refers to the Christians who have died before the return of Christ. They will rise first and return with Him (2 Cor. 5:8). They are "in Christ," that is, they are saved. The phrase "in Christ" is a designation of being saved, along with other great things like redemption, eternal life, no condemnation, etc. Check out "In Christ Grid."
1 Thessalonians goes on to say . . .
"Then we who are alive and remain shall be caught up together with them in the clouds to meet the Lord in the air, and thus we shall always be with the Lord. 18 Therefore comfort one another with these words," (1 Thess. 4:17-18).
As you can see, there are two resurrections in this context. Those who "rise first" are resurrected to their glorified bodies before those who are raptured.