The command clears all configuration of the router and sets it to the default including the login name and password (‘admin’ and no password), IP addresses and other configuration is erased, interfaces will become disabled. After the reset command router will reboot.
If the router has been installed using netinstall and had a script specified as the initial configuration, the reset command executes this script after purging the configuration. To stop it doing so, you will have to reinstall the router.
[admin@MikroTik] > system reset
Dangerous! Reset anyway? [y/N]: n
(from mikrotik dot com)