A content freeze is usually set in place during a migration process, or to prepare changes in content which are not yet meant to go live.
All projects are set to be automatically published each night. To enable a content freeze for your project, your project Admin can turn off the automatic publishing in the Admin Portal, under the publish settings.
Note: if the automatic publish has been switched off, the publish button will still be available to push the project live manually. If you would like to have this button disabled during a content freeze, your Admin will also need to disable Publishing.