These gems allow light to pass through but are not transparent. It is not only the color of the stone that causes translucency. There are stones that are naturally translucent, but in the case of the grade scale this refers to stones that have such a high number of inclusions or blemishes that are highly visible they stop the light from passing through the stone.
Emeralds are naturally a Type 3 colored gemstone. That means they are naturally blemished and flawed. If you were to compare two stones, one being a higher quality stone with less inclusions and the other being highly flawed, you will see a visible difference in brilliance. That means how each stone shines as light passes through. The high quality stone will be brighter and more attractive, while the lower quality stone will appear dull. That is what translucent means in terms of grading a gemstones clarity.