Backtest
A backtest applies a strategy to historical data to measure how it would have performed: gains, losses, drawdown.
A backtest can be manual (you scroll the chart candle by candle and log each trade) or automated, using the MetaTrader or TradingView strategy tester or a programming language.
For it to be worth anything, the rules must be written without ambiguity, the sample must be large enough (several hundred trades if possible, across different market conditions) and real costs must be included: spread, commission, slippage, swap.
A backtest does not prove a strategy will win in the future. It helps you weed out bad ideas and learn a method's characteristics, such as its typical drawdown and losing streaks.