This is the resurection of the dead in Christ. Those that since the Gospel has been preached, some 2000 years, have listened and accepted. They are part of the followers of Christ but have died before the return, or rapture. When the rapture, calling up, snatching out of the Christians takes place these Christians who are dead are also called up at the same time any they as one unit or church meet with the Lord in the air, forever to be with Him.
As to their whereabouts during their death we can not say only that the Lord tells us that to be absent from the body (in death) is to be present with the Lord. We can only assume that a place called paradise exists, in the form of a garden perhaps, as this is the literal translation, or some intermediate place where they can relax awaiting the call. However we must always remember that when we die we enter eternity and as such time ceases to be so a thousand years on earth, living could be only a few seconds relatively speaking in the spiritual world.