What does it mean by software's product ID?

Answer 1

That's usually the serial number that comes with it. It's sometimes on the cd case and other times its on the back of the cd's instruction booklet inside the case. On computers, the OS product id on a sticker on the case most of the time.

Answer 2

while buying an original software u r provided with a Number/ word( or Combination) from 5-6 digits to 20-30 digits/words. without this u can't install , operate or upgrade that particular software.
this is also used to verify the originality of that software.