it runs a Monte Carlo type analysis on programme activities. It allows you to set up Max, Min and Most Likely durations for each activity then schedule the programme several thousand times using "random" durations based on the Max, Min, Most Likely figures that you input.
The completion dates are recorded and you can establish what percentage completed by date x, and what percentage completed by date y etc. This allows you to estimate a probability of achieving a certain date.
It is difficult (time consuming and complex) although not impossible to use on a large construction project, and is useful for maintenance shutdowns or disruptive works on the railway where you wish to demonstrate, say, a 95% probability of finishing on time. If you do use it on construction projects it is a useful tool to show your client the 90% probability (safer) dates, whilst driving to achieve the 80% probability (harder to achieve) dates.
I hear people thinking "garbage in, garbage out", but it has been surprisingly accurate where I have worked with it.