An opaque surface is one you can't see through. If you shine your flashlight on a smooth, flat, opaque surface such as a book, the light will be reflected. This means the light bounces off at the same angle that it hits the smooth flat surface.

A transparent surface is one you can see through. If you shine your flashlight on a transparent surface, such as water, some of the light is refracted. This means that the light bends when it enters the water. Water is more dense than air. Its particles are packed together more tightly than the particles in the air. This makes the light bend. Refraction occurs when light slows down as it enters a more dense material.