This dialog is accessible through menu Edit / Revert.
When choosing that menu option, the dialog appears with the book and chapter that you work in selected.
Bibledit displays all the dates and times that a changes was made in that chapter.
It also shows the current text, that is, the text as it is at present.
If you double-click on a revision, or press Enter, then the text as it was on that date and time is loaded as the "previous text".
If there were any changes between that date in history and the current text, that will be shown under "changes".
Lines that were added since that date in history are shown in bold.
Lines that were deleted since that date are shown in strikethrough.
If you press button OK, Bibledit will revert to the "previous text".