In the story, Mrs Who and Charles Wallace explain to Meg that they will be traveling by "wrinkling time" through a tesseract and that "the fifth dimension's a tesseract. You add that to the other four dimensions and you can travel through space without having to go the long way around. In other words, to put it into Euclid, or old-fashioned plane geometry, a straight line is not the shortest distance between two points."
In mathematics a tesseract is a four-dimensional shape (hypercube) that when represented in three dimensions looks like a cube inside a cube with spokes connecting the corners of the two cubes together. In the novel, the tesseract functions more or less like what in modern science-fiction is called a space warp or a wormhole, a portal from one area of space to another which is possible through the bending of the structure of the space-time continuum.