Draw from a swing low to a swing high and the tool plots percentages of that move. The 50% and 61.8% levels are the most watched. There is no proven mathematical reason prices respect them; they work partly because enough traders watch them and partly because pullbacks of about half a move are common anyway.
Use them as zones for confluence with support, a moving-average, or volume-profile, not as precise targets.
Example: a stock runs from $100 to $140. The 61.8% retracement is $140 - (0.618 x $40) = $115.28, and 50% is $120. A pullback into $115 to $120 that holds is the kind of setup swing traders watch.
Related: pullback, confluence, support, trend