Flat space refers to a geometric concept where the rules of Euclidean geometry apply. In flat space, the angles of a triangle add up to 180 degrees, and parallel lines never meet. This type of space can be visualized as a two-dimensional plane, like a sheet of paper, where distances and shapes behave predictably. In contrast to curved spaces, such as those described by Albert Einstein's theory of general relativity, flat space is simpler to understand and calculate. It serves as a foundational concept in mathematics and physics, helping to explain more complex structures and the behavior of objects in our universe.