It means that 8% of the taken in energy (in this case elevtricity) was converted into useful energy (remember that energy cannot be created or destroyed, only changed). In this case, 8% of the taken in electricity was converted into light, which is the useful energy conversion as the purpose of the lamp is to provide light.
The other 92% of the electricity was wasted. This means that it was converted into energy that was not useful for the purpose of the product (remember, energy can never be destroyed, only changed). In this case the other 92% is likely to be heat.