Once you have purchased your gift card. You will receive an Email within 24 hours with a unique code on it which will be for the value of your voucher.
Attached to the email their will also be a document which is able to be printed off which will have the value of the gift voucher and the code displayed on it. You will also have the option to add the voucher to your apple wallet.
Every gift voucher is valid for in to a year from date of purchase and can only be used once. The date of purchase will be displayed on the document attached to the email.