This command allows for temporary changes to the services configuration. These changes are not written to disk and are lost if services is restarted or stopped for any reason. Connections to services that are interrupted for some reason will not cause these changes to be lost assuming the other conditions do not happen. To make changes to the configuration permanent, you will need to edit the configuration files on disk, save them, and use a reload command.
This command accepts at least 1 parameter, either the word modify or view. The word modify is then followed by the modifications you want to make. You can use view to see what you want to change, and then use modify to change it. For example, to turn off NickServ's forcemail option, you can use this command:
/msg OperServ config modify nickserv forcemail off
Usually, changing configuration files and reloading them is preferred to using this command, as the latter is permanent and easier.