The O'Keefes are an unusually gifted family and the primary characters in A Wrinkle in Time by Madeleine L'Engle. After their father has been away (missing) for some time, Meg and Charles Wallace plus their friend Calvin meet some rather odd but pleasant women named Mrs. Which and Mrs. Whatsit. They explain to the two O'Keefes that their father is trapped in some kind of darkness, and it is imperative that they go to rescue him by tessering--time travel. And then they do it.
Meg is confused, and her brother explains that there are not four but five dimensions in the universe. The first dimension, of course, is a line; the second is a square, the third is a cube, and the fourth is time. The fifth dimension is a tesseract, and it is the theory of time travel their father was studying when he disappeared.
If time is a string (a straight line), an ant, for example, can shorten his traveling time by folding (wrinkling) the string to shorten the traveling distance. This bending, folding, or wrinkling in time is known as a tessaract.
“In other words, to put it into Euclid, or old-fashioned plane geometry, a straight line is not the shortest distance between two points.”
Virtually the entire novel involves time travel, and none of the characters could do it if it were not for tessering, as traditional time travel is not feasible.