Hi, Rebecca and welcome back!
I'm so happy to hear that you decided to purchase
Disney's PhotoPass+ for your upcoming vacation!
Inside of the disc holder, you'll find a disc loaded with beautiful, high resolution stock photos from the Walt Disney World Resort! The only time you should worry about the "void if unsealed" message is if you change your mind about your purchase. Once that disc has been opened, you would not be able to return it. You will not need to bring that disc with you on your vacation.
You'll definitely need to bring the voucher you received with you; however. That is how you will get your PhotoPass+ card upon your arrival to the Parks. Simply go to one of the PhotoPass Sales Centers located in each of the 4 major Theme Parks and Downtown Disney to exchange your voucher. You will receive one card on a lanyard so you'll want to be sure to keep that in a safe place while you're touring the Parks.
Thanks so much for stopping by with your question, Rebecca! I wish you a wonderful vacation filled with lots of memories captured on your PhotoPass!
Smile and say "cheese"!
~Jackie