An Account Identity on Polkadot refers to the way an account is uniquely identified and associated with additional information beyond just the basic account address. It provides a means to associate metadata or information about the account holder or entity within the Polkadot ecosystem.
To keep People’s details safe, they need to give some coins to Polkadot. It’s like giving a promise that they won’t take their details away. If people give extra details like their address or other stuff, they need to give a bit more coins. The coins don’t disappear; they’re just waiting until people want to take their details back.