systemsettings is the KDE system configuration tool.
Usually, it is started via the Kickoff Application Launcher, but you can also start with ALT+F2 and type 'systemsettings', or start from a konsole.
If it happens that you cannot start it, that none of the methods to launch the application work, without there being any apparent error (e.g. starting from a konsole gives no output, and nothing starts), it may be that systemsettings is already running in the background. Find it or find its pid, kill it if necessary and start again.