What part of the tesseract were Meg's parents working on before her father disappeared

Mrs. Whatsit explains the meaning of the tesseract. Using some illustrations and diagrams, Mrs. Whatsit shows them how they “wrinkle” time and space, making travel between great distances easy and quick. The tesseract is the fifth dimension of space, a dimension the children can travel through meaning that “a straight line is not the shortest distance between two points.” It was a concept Meg’s parents have been working on for a tremendous amount of time.