Okay the thing is that while generating random numbers care has to be taken that the numbers so generated dont repeat in a pattern or frequently (though this would happen if the range is small).
To safeguard from this, normally all random generators make use of a base values (normally a very large number) which serves as a base or as a reference so that the numbers so generated using them dont repeat.
This said, care should be taken that the reference number should be as unique as possible and should not be ideally repeated.
The time( ) function returns the number of seconds that have passed since 00:00:00 GMT January 1, 1970. So when this value is passsed to the time function, it serves as a satisfactory reandom number generator reference or seed value as they call it.
More information here.