We send you an email that contains your trial (or purchased) license key - this is the 16 digit key at the top of the mail.
The email also contains an "offline license" - this is the large block of letters and numbers further down.
When you start FlashBack for the first time you'll be asked to enter the license key. If FlashBack doesn't find a working internet connection, it will ask you to enter the offline license.
The offline license lets you run FlashBack without an internet connection for up to 7 days. After that it needs to connect to the internet and check its license with our servers. If you're using it on a PC that never connects to the internet, contact us at firstname.lastname@example.org.