When light hits the boundary between glass and air or water and air it is usually refracted out but as the angle of incidence is increased an angle is reached where the light emerges along the boundary - the angle of refraction is 90o.
As the angle of incidence is increased the angle of refraction also increases until the light emerges along the boundary between the block and the air.
The angle of incidence in the transparent material when this happens is called the CRITICAL ANGLE.
For angles of incidence greater than the critical angle ALL the light is reflected back - this is known as TOTAL INTERNAL REFLECTION.
Total internal reflection only occurs when the light is travelling from the more dense material to the less dense material (glass to air, glass to water or water to air).