Fibonacci retracement is a technical analysis tool used in financial markets to identify potential levels of price correction or retracement within a trend. It's based on the Fibonacci sequence and specific ratios, primarily the key levels of 23.6%, 38.2%, 50%, 61.8%, and 76.4%.  These ratios were introduced to Western Europe by Fibonacci, also known as Leonardo of Pisa. He was an Italian mathematician from the Middle Ages who introduced the Hindu-Arabic numeral system to Europe and is famous for the Fibonacci sequence. 

To apply Fibonacci retracement, traders draw horizontal lines on a price chart from the peak to the trough of a trend, or vice versa, creating potential support and resistance levels. These levels are believed to correspond to natural price behaviors and have significance in many financial markets.

Fibonacci retracement is used for several purposes:

Identifying Support and Resistance: Traders use Fibonacci levels to anticipate where an asset's price might find support during a correction or encounter resistance in an uptrend. The 38.2% and 61.8% levels are especially significant.

Entry and Exit Points: Traders can use Fibonacci retracement levels to determine optimal entry points to buy or sell an asset within an existing trend.

Setting Stop-Loss and Take-Profit Orders: Fibonacci retracement helps in setting stop-loss orders to manage risk and take-profit levels to capture gains.

Confirmation: When other technical analysis tools, such as trendlines or moving averages, align with Fibonacci retracement levels, it can provide additional confirmation for trading decisions.

It's important to note that while Fibonacci retracement is widely used, it's not foolproof, and traders should consider it alongside other technical and fundamental analysis tools to make informed trading decisions.