Three Line Strike
Basically, three consecutive candlesticks by the same team, followed by one larger in the opposite direction. This patterns can take different shapes.
Bullish three line strike
Here the pattern is for three similar bull candlesticks in an increasing pattern, are followed by a massive bear candlestick. The bear candlestick engulf all three prior candlesticks.
Think of it like three small attacks from the longers, that are counter by a massive attack from the shorters. The shorters normally win and the prices go down.
Bearish three line strike
Here the pattern is for the shorters/bears to strike three times. See how in the image below there are three bear candlesticks, in a downward pattern, which is followed by an engulfing bull candle. The bulls win and the price goes up. According to Bulkowski, this reversal predicts higher prices with an 83% accuracy rate.
Here is another example: