What does it mean that I must reset my phone end to end data encryption

iCloud security overview - https://support.apple.com/HT202303

"End-to-end encryption

End-to-end encryption provides the highest level of data security. On each of your devices, the data that you store in iCloud and that's associated with your Apple ID is protected with a key derived from information unique to that device, combined with your device passcode which only you know. No one else, not even Apple, can access end-to-end encrypted information."

Related discussion by Apple Community Specialist --> https://discussions.apple.com/thread/252241993