Hello,
basically, the configuration register influences router behavior with regard to how the router boots (into ROMmon, or NetBoot), or what to do while booting (ignore configuration, disable boot messages), but also to set the console speed (baud rate for a terminal emulation session).
0x2102 is the most common setting.
http://www.cisco.com/c/en/us/support/docs/routers/10000-series-routers/50421-config-register-use.html
