What is the Tesseract?

In response to her questioning, Mrs. Which informs Meg that her father is trapped behind the darkness. Mrs. Whatsit assures her that they are traveling to help him. She explains that they travel by tessering, which involves taking shortcuts through time and space. Seeing that Meg remains confused, Charles Wallace explains that tessering is travel in the fifth dimension: the first dimension is a line; the second is a square; the third is a cube; the fourth is Einstein's concept of time; and the fifth is a tesseract. By adding the tesseract to the other four dimensions, they travel in such a way that the shortest distance between two points is not a straight line. Although Meg does not completely understand, she contents herself with this explanation.