In the diagram above, the street cleaner can still hear the sound produced by the radio even though the sound should be blocked by the corner of the building. However, he can't see the radio. This is what it mean by "sound can turn round at the corner of a corridor but light cannot".
Sound can turn round at the corner of a corridor due to diffraction of wave as shown in the diagram below. The magnitude of diffraction will increase as the wavelength increases. The wavelength of sound wave is much longer than the wavelength of light wave, therefore it is diffracted and reach the cleaner. The wavelength of light wave is very short. Therefore, light wave can't be diffracted to reach the cleaner.