Fantasmic is free with park admission, and a great show at that! Lots of people don't want to spend hours waiting in line to get great seats, so they book a Fantasmic dinner package so they can get reserved seating and not arrive as early. You pick a restaurant, eat there taht night, get a ticket that will allow you to sit in the reserved section, and show up more like 30 min prior.
I havent done the package because I've never gone when it was busy enough to wait hours on end for seats. Many say its worth it, esp with small children who cant stand sitting for so long.
=)