This command causes the channel to be synced with the information that services has on files for the various users. Channel modes are set by services to match users and channel settings based on things such as access list entries or the channel's mlock. Services will normally handle all of this for you automatically, so it is expected that you will rarely need to use this command.
This command accepts 1 argument, the channel to be synced. To sync the channel #channel, you would issue the following command:
/msg ChanServ sync #channel