Fibonacci retracement
Ratios (38.2%, 50%, 61.8%) that measure how much of a move has been retraced; used as possible support/resistance zones in pullbacks.
Fibonacci levels aren't magic; to the extent they work, it is because many people watch the same levels and because they make deep and shallow pullbacks measurable. A 61.8-78.6% retracement is "deep", 38.2-50% is "shallow".
A level is not an entry on its own; it is used together with structure, an order block/FVG and candle confirmation. Fibonacci extensions (127%, 161.8%) are used to set targets.