im just want to know how actually "random function" on arduino work .?
https://www.google.com/search?q=pseudo+random+number+generator
https://www.google.com/search?q=minimal+standard+random+number+generator
how it can randomly choose something ?
It can't. The function gives the appearance of being random. For most applications that is good enough.
(example we set "random(1,9)" and it can randomly choose number from 1 - 9)
One to eight. The maximum value is exclusive...
http://arduino.cc/en/reference/random
The Arduino
random functions are simple functions layered on top of the Libc
random function. Values are mapped from 0 to RAND_MAX using modulus...
https://github.com/arduino/Arduino/blob/master/hardware/arduino/cores/arduino/WMath.cpp#L29