In a normal hearing course, earlap (tragus) just like a dish antenna collects the sounds from surrounding area that reaches us and transmits to eardrum via external ear. Sound waves reaching to eardrum is vibrated by 3 pieces (ors, stapes, malleus ossicles) little ossicles between eardrum behind the eardrum and inner ear and transmitted to inner ear. On this way sound waves are strengthen by eardrum and ossicles and transmitted to inner ear. 

Any malfunction on this way; Plug accumulation on external ear canal, calcification in the eardrum, eardrum torn, fusion-splitting-calcification of ear ossicles, infected or not infected liquid accumulation in middle ear. In the course of these conditions sound waves reached to earlap cannot be transmitted to inner ear as strong as it is required. And this is called conductive hear loss.