Group Policy settings ultimately manifest as values within the Windows Registry, primarily under HKLM\Software\Policies (Machine) and HKCU\Software\Policies (User). When /force is executed, the Group Policy Client essentially wipes these registry hives and rewrites them according to the current Active Directory GPO state. This ensures that any "tattooed" registry changes (manual changes made outside of policy) are overwritten, restoring strict compliance.
If your computer is part of a Domain (common in office environments), it needs a stable connection to the Domain Controller to pull the latest policies.
During a standard refresh: