A tesseract is a fictional, higher-dimensional space-time construct that allows for instantaneous travel across vast distances. Mrs. Who uses the sheets to create a "wrinkle" in the fabric of space-time, folding it and allowing for a shortcut through the universe.
The sheets are not simply a prop, but a necessary component in the creation of the tesseract. It is through manipulating the sheets and creating a specific pattern that Mrs. Who can access this higher-dimensional space.
This scene highlights the importance of imagination and creativity in the story, as the characters use seemingly mundane objects to achieve extraordinary things.