A rubber band: the harder you stretch it, the likelier it snaps back. But how much further it can stretch, you can't tell in advance.
RSI compares how much the price gained on rising candles and how much it lost on falling ones over the last 14 candles. Above 70, traders say “overbought”, below 30 — “oversold”. In the simulator RSI is drawn below the chart, with lines at 70 and 30.