To my knowledge:  the  APAP is Tylenol and the 5 is mg of Vicodin and the 500 is mg of Tylenol.  I think you can go to WEBMD and ask or search it.  Hope this helps.  I take this, but I never really asked this question specifically.  LOL.  I just took the general idea of it. 

Great question because you should always understand what medications you are taking. The top number is the opioid, in this case hydrocodone (5mg). The bottom number is the APAP (Tylenol) 500mg. Always, always, pay attention to how much Tylenol you use in a 24 hour period of time. Tylenol will kill you if taken at high amounts.

The maximum amount of Tylenol for a healthy individual is 4000mg per day(For 5/500 that is eight per day). If you have liver problems like hepatitis, please consult with your doctor before you take Tylenol. Thanks for paying attention.

Dr. Christina Lasich, MD

If you have any liver dysfunction, consume alcohol, or a number of other drugs, the safe limits on acetaminophen are much much lower per day.  There are other preparations with low dose acetaminophen such as Norco (325mg acetaminophen per tablet) and a few others...   Hydrocodone may be obtained in the pure form without acetaminophen from a compounding pharmacy in Schedule III doses (10mg or less), Schedule II doses (15mg or more, either sustained release or instant release).

I am not sure what apap means but the 5 means the amount of hydrocodone in milligrams and 500 is the amount of acetaminophen in milligrams.