The Bullish Doji Star is a bullish reversal pattern represented by two candles.

During a downtrend, the first candle is decreasing and has a long body. It is followed by a Doji that opens and closes below the previous candle.

Dojis are indecision patterns and represent how bulls and bears fight to determine the future direction of the price. 

This pattern represents how a downtrend fades out and leads to a bullish reversal. The Doji at the end of the downtrend reflects how the bulls have suddenly appeared to stop the trend and, very likely, push the price up

Since this pattern usually precedes to rises in the price, it will signal a buy every time that it appears in the chart.