An annular fissure is basically a tear in the annulus fibrosus layer (ie. The outer layer of a disc). It means that some of the inner spongy layer (ie the nucleus pulposus), can squirt out through the torn fibers of the annulus fibrosus. Just like the pulp of an orange can squeeze through the outer peel. When you have a ‘tear’ in this outer disc layer (ie. Annular fissure), this is when the inner nucleus pulposus ‘herniates’ out and becomes a problem.
Remember the annulus fibrosus is made up of about 25 sheet layers. So a tear can start off on the inside as a ‘contained’ fissure, just within the inner layers of the annulus fibrosus, but overtime can spread so that the tear breaks through all approximate 25 sheet layers and allows the inner nucleus pulposus to ‘herniate’ outwards onto the surrounding nerves.