My Friend recently gave me her old iPhone 6 and she said that it was currently iCloud locked to AT&T. I have no clue what an iCloud locked AT&T iPhone means. Can I still use the iCloud Locked AT&T iPhone 6 as an iPod to store some music on?
iCloud lock is properly called Activation Lock and doesn’t have anything to do with any carrier:
The owner of that iCloud can release the device from any web browser they log in to their account.
If the phone is carrier locked - you’ll need to work with your new carrier or the old carrier to resolve that if you want to have cellular data on it.
With that out of the way - you can always use a carrier locked iPhone as an iPod Touch and no data connection. That lock only prevents cellular data and service. You can’t use an Activaytion Locked device since iTunes won’t restore it and Apple won’t sign an download to run the OS on a locked / presumably stolen device.