Monte carlo simulation just means you ran many different possible scenarios(hundreds to potentially millions) so you can look at a range and see where the results tend to congregate(a mean). They use it in many different fields. In financial planning it is normally used for running projections of what the future could look like.
Whether it is reliable depends on the inputs.
Some monte carlo simulators just use historic data. The upside is that it captures various relationships(stocks Vs bonds Vs inflation, etc.). The downside is a very limited amount of projections. If you run 30-40 years of possibilities you only have so many backtesting scenarios to use.
Pure monte carlo: This normally looks at the mean and standard deviation of different investments. You can run this type of simulation infinite times. The downside is that it normally fails to capture the various interrelationships and you can end up with some crazy potential results. If you run 10,000 scenarios, 1 could show your money growing 100%/yr forever.
What you want is something that takes the second, and based on a lot of research tunes it to be more reliable/realistic. The problem is this will probably require a lot of analysts/mathematicians so it can be expensive. The big investment firms have this sort of thing. So if they ran the projections for you I would put more weight on it than if you just used a free pure simulator or if you just used historic data.