You can set up a screen lock to secure your phone. Each time you turn on your phone or turn on the screen, you'll be asked to unlock it.
Tap Screen lock and select the type of lock you want.
Some lock options aren't available if you added a VPN or a work (corporate) email account to your phone.
No lock:

Standard lock:
Biometrics:
You must set up one of the 3 standard locks in order to use biometrics.
If you're using a PIN, password or pattern, you can:
Important: this leaves your phone with no protection. Anyone who picks it up can access your apps, messages, emails, photos and any sensitive information on it.
If you forgot your PIN, pattern or password, try these troubleshooting steps.