Answer:
This is from 1 Thessalonians 4:16-17 For the Lord himself shall descend from heaven with a shout, with the voice of the archangel, and with the trump of God: and the dead in Christ shall rise first: Then we which are alive and remain shall be caught up together with them in the clouds, to meet the Lord in the air: and so shall we ever be with the Lord.
Today, when a Believer dies their spirit and soul immediately goes to be with the Lord in Heaven but their physical bodies will not be resurrected until the return of Jesus Christ for His bride (the church or body of Believers). The verses in 1 Thess. talks about this and what will happen to those bodies in the grave whose spirit and soul are already with the Lord. Burkitt in his commentaries states the following:
The blessed consequence of his coming, namely, the resurrection of his saints: the dead in Christ shall rise first; the saints shall rise with the very same bodies which they laid down in the grave; and they shall rise first, that is, before those who are alive shall be changed: though it is very probable that there will be no considerable difference of time between the glorification of the raised saints, and those that are alive at Christ’s coming, yet it seems evident that the dead saints shall be raised, and in their bodies be glorified, before them that are alive shall be changed.
I hope that helped to explain. Good question!