Those Christians who adopt the Mid-Tribulation Rapture position believe that Jesus Christ will return for His children sometime during the Great Tribulation, as opposed to those who believe that He will come before this period begins, or after it ends. For reasons which are fully explained in his in-depth series,
The Great Tribulation And The Rapture, Bill Kochman currently believes that the Post-Tribulation Rapture is the most Scripturally-sound view of the various Rapture positions. Please read this in-depth series for a more-detailed explanation.