It shows the dead and living IN CHRIST being "caught up" (snatched away, Raptured) before the 7 year Tribulation. Scripture in the Tribulation Map explains this event. It will come in the "twinkling of an eye," so quick, people will not have time to park their cars. World wide panic and havoc will probably occur. This picture is a powerful witnessing aid.
See "Interesting Points about the Rapture Picture" PDF