Monte carlo simulation is a statistic method not only related to electronics, but more in general is used to investigate phenomena in which there are many random variables varying following a known distribution (uniform, gaussian, etc.), like, in electronic the tolerance of components, substrate thickness variability, etc.
As said by Prashanth.vinnakota, the simulator will generate many runs, each having a different value for the set of random variable (however inside a defined range of variation). Each set will be generated so that all the variable will be uncorrelated each other (usually a chi-squared test is used to check the condition). At the end of the simulation the result of all the runs is presented in text or graphical format.
The results will represent the behaviour of the system taking into account the variability of the parameters.
If a large enough number of runs is launched then the worst case (or almost worst case) can be identified.