What doesTime hollow from the book found by Margaret Peterson Haddix mean?

The first time that they go into the time hollow (the day, I don't know because my language arts teacher hasn't returned it to me yet,) is when Jonah and Catherine's parents bring them and Chip to a convention of some sort for the invited adopted children, who, like chip and Jonah, are missing children from history. they then go on a "hike" with the "counselors" (Gary and Hodge), and go up into what looks to be a cave. this cave is the time hollow, Gary and Hodge's initial attempt to finish what they started, to get the kids to the future. their plan is then foiled by none other than J.B.