A Swing Low is a 'valley' identified within a series of bars or within an indicator or oscillator. A Swing Low consists of two "legs", and starts with an initial down leg which is then followed by an up leg. A leg is defined as two or more bars going in any one direction. For example, when two or more consecutive bars produce lower (bar) highs and lower (bar) lows. In this instance, the smallest possible down leg would have developed. Inversely, to have an up leg you need no fewer than two bars showing higher (bar) highs and higher (bar) lows. When a down leg is followed by an up leg, the valley that is created is considered a Swing Low.
Background: A Swing Low starts its life out as a pivot low consisting of a high low to the left and right of a price bar. To additionally produce a formal Swing Low requires that the bar forming the price low has two bars with higher lows preceding it and two bars with higher lows following it.
Swing Lows can act as an area of support or resistance.
Practical use: Defining a Swing Low is helpful to technical analysts because many trendlines are drawn connecting the peaks of Swing Lows to each other. This is also helpful to begin gaining a deeper understanding of the sentiment read of a chart.