That is the password that is setup and used for an encrypted backup. This password is created just before you begin the backup process and choose the encrypted method.
My recommendation is that you connect your iPhone to your Mac again and then delete that first backup by choosing Manage Backups and then Right Clicking on that backup and choosing Delete.
After that, you can start a new backup. If you choose Encrypt Local Backup then you will be prompted for a password. Keep it unchecked if you don’t want to encrypt the backup, however, note that none of the passwords to the apps will be saved in the backup.
Axel F.