Phantom is a non-custodial wallet and browser plugin that may be used to manage digital assets and exchange data with smart contracts and digital apps. Being non-custodial means that, as opposed to having your assets maintained in one place, you still hold a limited amount of private documents. Just as you would need to keep your real wallet or purse safe from pickpockets when you are out and about, you are also accountable for keeping the contents of your Phantom wallet secure. This is not the same as keeping the currency of your nation in a bank account.
Phantom Wallet