When there are two media, say glass or water at the bottom and air at the top, then a light is directed into the glass at the bottom, and shines up, it will emerge into the air. But if the angle of the incident ray is changed, at a certain point the light will not go into the air. It will go along the inside of the top of the glass, and some will be reflected back inside the glass. This angle between the ray and the normal is called the critical angle.
First I will show you the prism. Light is going to come in here through the curved side. I am going to move the prism round. You will see when the light disappears, and doesn't emerge any more. We will turn the light on again then to see what the angle was. Let's have the lights off now please.
Did you see the light disappear as I changed the angle of the ray of light on the prism? The critical angle was 42 degrees. With glass and air, it's always 42 degrees.