One of the settings in Signal that you need to adjust is called ‘Registration Lock’. This means that you need that PIN to install your Signal account on a new phone so that no one can hack your account. If someone remotely accesses your smartphone, they won’t be able to see your message history that way. If you don’t enable that setting, your phone number or your SIM card is sufficient for access.
Another important setting that is not automatically on is the ‘App lock’. With that, you need the unlock method of your smartphone to access Signal. This can be your facial recognition (Face ID on iPhone), your fingerprint or a code.
Finally, the setting ‘Viewing prevention’ also provides extra security. By turning that on, you prevent other applications from taking print screens from Signal. The app does this by not showing its content in the list of recently opened apps.