Conventional bombs cannot be dropped below a certain safety altitude. This is because in low altitude flight the bomb would hit and explode before the aircraft had a chance to depart the dangerous fragmentation zone. In order to be able to drop bombs in low altitude flight, bombs with air brakes were invented. These bombs open air-braking surfaces or inflatable Ballute, which significantly increase air resistance and thus reduce the bombs forward speed during its fall. This delays the impact and thus also the detonation allowing the attacking aircraft enough time to clear from the fragmentation zone.
Here is a picture of a Mk82b Snake Eye bomb with retard surfaces:
To use the retard feature of the bombs, you have to activate them as follows: