Monte Carlo: what is a seed?
The seed is an initial number of the pseudorandom number generator(PRNG) which is in fact fully deterministic. It returns a sequence of numbers that looks random enough for many purposes, but always generates the same sequence for given value.

In order to make simulation non-repeatable (returning always the same value) they advice different initial value for every simulation (this is what non-fixed seed means).


Image taken from here