Controls the positions of vertex normals. This affects the appearance of shaded polygons.
By default, Maya locks normals to their existing values.
Regardless of the current normal’s values, clicking this button sets the normals to unit length (or normalizes them).
The unit length of a normal is calculated by the point offset from the vertex. You may inadvertently change the X, Y, Z values to ones that change this unit length to something undesirable which could un-normalize the normals (for example, changing X to 1.11).