The Polkadot network has set an existential deposit requirement (also called ‘ED’) to make sure every existing Polkadot address is live and active. According to the official explanation of ED , the minimum amount of DOT held in a wallet cannot be less than 1 DOT (and 0.0000333333 KSM on the Kusama network).
The doc also mentions that “If an account drops below the ED, the account is deactivated (“reaped”) and its funds are destroyed. The address can only be reused by reactivating it with a new deposit over the minimum amount. This will not restore the destroyed funds. ”
Under this mechanism, in order to prevent your account from going below the ED, users are not able to execute a transfer that will leave the balance below the ED. When you try to do so, there will be an error pop-up in the App showing the possible outcome of the action.