If you own crypto on Exchange, Binance and more. You don't have the keys, so you don't have control over your cryptocurrency. Already in self custody, keeping a cold or hot wallet, you have your keys and have full control of the cryptocurrencies. However, it can be difficult for a lay person to properly understand and keep their keys. So that's what Vitalik proposes, to change, to do something like you sign in to a Google account, for example.