So if you have a ray coming from glass into air, any ray that is approaching the glass/air interface with an angle of incidence less than the critical angle (look it up - I think it is about 60 degrees in this e.g.) I.e. closer to the normal at 90 degrees, will go through out into the air i.e. will undergo refraction.
Conversely, a ray approaching the interface with an angle of incidence greater than the critical angle i.e. getting towards parallel to the interface and further from being at right angles, will be totally internally reflected.
The latter is the principle used in optical fibre technology as in broadband and in medical instruments e.g. gastro-oesophago-duodenostomy.
Hope this helps!