what does error 1015 mean during iPhone restore?

Error 1015 is most often given as a downgrade Error. This is most often due to the iPhone having the iPad baseband 6.15.00 installed. This 6 series baseband is higher than the current 5 series basebands. So when iTunes tries to install the current 5 series baseband, it is superceded by the installed iPad 6 series baseband, thus iTunes is attempting to downgrade the baseband to the 5 series, which Apple will not allow. This not allowed try at a downgrade stops the Restore process and results in the Error 1015. This error is now completely fixable, you must search the Web for the fix. The fix will allow future normal Restores with iTunes and will fix the GPS chipset which is compromised by the iPad baseband.