Raveling occurs when the stone aggregate that was originally part of the pavement begins to break free from its bonds with the asphalt. Typically this tends to occur on older pavements that have already oxidized.
Over time as more and more aggregate breaks free from the asphalt, the asphalt loses significant load-bearing capability and will begin to prematurely fail in the areas that have exhibited the most raveling and bears the most traffic-loading.
The typical repair for this type of situation is to overlay the raveling asphalt with a new layer of fresh asphalt. Typically 1.5" to 2" of new asphalt is recommended.