From Traderpedia

The Dragonfly is a type of doji that forms with its open, close and high at the same level. It is a candlestick that emits bullish properties and can be significant when found at that the bottom of a downtrend, especially if this is near a level of support.

The Dragonfly should have a long lower shadow, which indicates that the market moved significantly lower during the session, however the bulls exerted enough buying pressure to force the price back up to the opening level.

The Dragonfly is closely related to the Hammer which is also found at the bottom of a downtrend and can signify the start of a reversal.