Imagine that a friend is asked to draw on the front face of a blank sheet of paper. How could you determine which is the front face of a blank sheet if both sides are equal? This is why Normals exist.
A Normal corresponds to a perpendicular vector on the surface of a polygon which is used to determine the direction or orientation of a face or vertex.
In Maya, the object Normals are visualized by selecting the property Vertex Normals. This allows us to see where a vertex points in space and determines the hardness level between the different faces of an object.