A 4-manifold is a mathematical space that has four dimensions, meaning it can be described using four coordinates. Just like a 2D surface can be flat like a piece of paper or curved like a sphere, 4-manifolds can have various shapes and structures. They are studied in the field of topology, which explores properties that remain unchanged under continuous transformations. One of the most famous examples of a 4-manifold is the K3 surface, which is a complex, smooth surface with unique properties. Researchers use 4-manifolds to understand complex systems in physics and geometry, including theories related to string theory and the shape of the universe.