Strong Device Passcode
iPhone Device & Theft Protection
Essential
Use a strong, unique iPhone passcode that no one else knows.
What this setting does
Your iPhone passcode protects access to the device and the information stored on it. Setting a passcode also enables iPhone Data Protection, which encrypts device data. Because an unlocked iPhone is also a trusted device for your Apple Account, keeping the passcode secret is an important part of protecting both the device and your account.
Important notes
Use a passcode that is unique to your iPhone and not known by other people. Apple supports six-digit numeric, custom numeric, and custom alphanumeric passcodes; custom numeric and alphanumeric options can provide stronger protection than shorter numeric codes.
If you believe someone knows your passcode, change it immediately. On current versions of iOS, the previous passcode may remain usable for up to 72 hours to reset a newly changed passcode. You can disable that capability by going to Settings → Face ID & Passcode → Expire Previous Passcode.
On iPhones with a Home button, this setting appears under Touch ID & Passcode instead of Face ID & Passcode.