The file /etc/rc.d/rc.local is executed at boot time, after all other initialization is complete, and whenever you change runlevels. You can add additional initialization commands here. For instance, you may want to start up additional daemons, or initialize a printer. In addition, if you require serial port setup, you can edit /etc/rc.d/rc.serial, and it will be executed automatically at boot time.
The default /etc/rc.d/rc.local simply creates a nice login banner with your kernel version and machine type.