In practice
Forward testing checks whether backtest results survive real time, where candles form slowly and the right side of the chart is unknown. Traders run the strategy on a demo account or with very small size and compare the results with the backtest.
It also tests the trader: whether the rules can actually be followed at the pace of a live market.
What to watch for
Demo fills are better than real ones, and trading without money at risk feels different. Treat the first real trades, at small size, as the last stage of the test.