randomSeed() initializes the pseudo-random number generator, causing it to start at an arbitrary point in its random sequence. This sequence, while very long, and random, is always the same.
If it is important for a sequence of values generated by
random() to differ, on subsequent executions of a sketch, use randomSeed() to initialize the random number generator with a fairly random input, such as
analogRead() on an unconnected pin.
Conversely, it can occasionally be useful to use pseudo-random sequences that repeat exactly. This can be accomplished by calling
randomSeed() with a fixed number, before starting the random sequence.
seed - number to initialize the pseudo-random sequence (unsigned long).