This is the complement of the getBookmark() method. It takes the result of the getBookmark() method that would have been stored in a variable. The TextRange start and end points are then reset to the value that was originally defined when the bookmark was recorded.
The Boolean value true is returned if the bookmarked locations could be restored and a Boolean false value if they couldn't. It really depends on what has happened since the bookmark was captured as to whether it can realistically be restored or not.