https://www.investopedia.com/ask/answers/05/fibonacciretracement.asp
A Fibonacci retracement is a popular tool among technical traders. It is based on the key numbers identified by mathematician Leonardo Fibonacci in the 13th century. Fibonacci's sequence of numbers is not as important as the mathematical relationships, expressed as ratios, between the numbers in the series.
In technical analysis, a Fibonacci retracement is created by taking two extreme points (usually a major peak and trough) on a stock chart and dividing the vertical distance by the key Fibonacci ratios of 23.6%, 38.2%, 50%, 61.8%, and 100%. Once these levels are identified, horizontal lines are drawn and used to identify possible support and resistance levels.