This policy setting allows you to configure the device lock (PIN or password) expiration timeframe. After the password expires ...

This policy setting allows you to configure the device lock (PIN or password) expiration timeframe. After the password expires the user is asked to enter a new password.  If you enable this policy, you can specify the number of days after which the device password will expire. After this time, the user is prompted to renew the password.  If you disable this policy then the user is able to have the same password indefinitely.  If you do not configure this policy, then existing device-specific settings that control password expiration are applied.