A normal is an imaginary line drawn at 90° to a surface. It is usually represented as a dashed or dotted line and all angles of incidence, reflection and refraction are always measured between the ray of light and the normal to the surface at the point of incidence.
For a ray of light striking a glass surface as shown
the angle of incidence i would be the angle between the ray and the normal to the surface at the point A.
Angles of incidence can vary from zero (when the ray and the normal to the surface coincide) to just less than 90° when the light grazes the surface.