Fibonacci retracement
Fibonacci retracements draw levels at 23.6%, 38.2%, 50%, 61.8% and 78.6% of a move to spot where a pullback might stop.
You draw the tool from the low to the high of a bullish impulse (or the other way in a decline). The platform then shows the retracement levels. The 38.2% and 61.8% are the most watched; 50% is not a Fibonacci ratio but is almost always added.
These levels work partly because so many traders watch them. They carry more weight when they line up with old support, a moving average or a round number.
For targets, extensions (127.2%, 161.8%) project the move beyond its end point. In every case these are zones to watch, not prices to buy blindly.