Fibonnaci was a famous mathematician who discovered what is now known as the Fibonnaci Sequence of Numbers.
In the world of trading these special numbers are used to predict the most likely areas for the price to pull back to after a larger move.
We suggest reading these guides for a good foundational understanding:
https://www.babypips.com/learn/forex/fibonacci-retracement
https://www.investopedia.com/terms/f/fibonaccilines.asp