its the angle light hits a medium. your probly doing it in fibre optics.
if a lights angle of incidence is less than the critical angle, it is refracted, else it is reflected.
ill try draw a diagram
___________ <--- the fibre optic border
......./|\
....../.|..\
...../..|...\
just say that was the critical angle (the angle which is not refracted nor reflected). If the angle of incidence (the angle at which the light touches the fibre optic border) is greater than the critical angle (i.e. if the light comes in at an angle to the right of the critical angle) it can be totally internally reflected. If it incidences between the critical angle and the straight line (i.e. the normal to the border of the fibre optic) then it is refracted and allowed to pass out) It is not reflected