The mirror of a swing-high. A three-bar swing low is a bar with lower lows on both sides; wider definitions require several bars each side and produce a cleaner, sparser set of points.
Swing lows are where uptrend trendlines are anchored, where stops for long positions commonly sit, and where a break signals structural change. In an uptrend, an intact sequence of rising swing lows is the simplest possible definition of the trend still being in force.
Like swing highs, they are confirmed only in hindsight. A trader watching live cannot know whether the current low is the swing low until the market proves it by moving away, which is why entries at swing lows usually require an accepted amount of drawdown or a confirming pattern.
Related: swing-high, market-structure, trendline, invalidation-level, higher-highs-higher-lows