Self-custody is a fundamental aspect of web3. It implies that to truly own your cryptocurrency, you must store it in a wallet that you have direct control over, rather than relying on a third-party exchange. When setting up your Metamask wallet, you will receive a seed phrase (or private key) that gives you access to your funds. If you store your crypto on an exchange, the exchange holds the keys and has control over your funds. In the event of any problems, your funds may be at risk, as you do not have full ownership of them.