A random number generator requires a number(it is called seed) to generate random numbers . If the random number generator is given the same seed then every time it will generate the same sequence of random numbers . For example :-
If you run the program and it is generating random sequence 2,78,45,60 . If second time you run the program you will again get the same sequence 2,78,45,60.
srand function is used to change the seed of the random number generator.By setting srand(time(NULL)) , you are setting the seed of the random number generator to the current time.By doing this every time you run the program you will get different random sequences :-
For example for the first run if you are getting 2,78,45,60 . Next time you might get 5,3,6,80 (depending on the current time,as seed has been changed since the time has changed since the last run)
for more info refer these :-