What password is this referring to?

I recently got a new iPhone. I backed up my old iPhone to my computer when my new phone said to connect to iTunes when I pressed restore from iTunes. Then, I hooked up my new phone to my computer and clicked,"Restore Backup." It asked me to choose a backup to restore, then I clicked my old phone, and it says to enter your password! I don't have a clue as to what "password" it's referring to, and I tried my computer user password, my old phone's lock screen password and my Apple ID password, but to no avail.

When you make a backup of iOS device using iTunes, you have the option to encrypt the backup with a password. Connect you phone to computer, run iTunes, in the Backup section, you should see an option "encrypt iphone backup".