Forward Test games out possible investment returns in the future, based on the risk of your portfolio and its components, by charting many possible paths and the likelihood of various outcomes.
The largest gains and losses tend to be the least likely, akin to the low probability for instance of repeated coin tosses which are almost all heads or almost all tails.
Forward Test also help you compare the effects of downside protection for a portfolio in limiting potential losses.
Forward Test is a form of Monte Carlo simulation, which can be helpful for making more informed decisions and managing risk, by considering a wide range of potential outcomes. Read more about the use of Monte Carlo simulation in finance at Investopedia and Wikipedia.