The 'Force Password Reset' can be used to force a person to reset their password on their next log in. This option can be useful if you have set a temporary password for a user and want them to reset the password to something unique on their next log in.
You can set this option by viewing the person in the Admin area, on their Profile page and the Access tab. Click the option to 'Force on next login' to enable.
NOTE: This option can also be set in the Upload People process by creating a column called 'Force Password Reset' and populating with Yes / No. This allows you to set this option during a bulk upload of new people.