<img src="http://stockcharts.com/education/glossary/Images/gloss-hammer.gif">
The price action that creates a candlestick of this shape indicates that the bears drove the price down during the day, but the bulls were able to mount a rally by the end of the day and the close was relatively strong. If the candlestick is white, indicating that the bulls were able to rally the stock above the opening level, then it is considered more bullish than if the close is below the open.
If this candlestick forms after a decline, then it is called a hammer. This is much like a washout day with a reversal -- The market is trying to hammer out a bottom. The hammer by itself is not a bullish signal, but rather serves to notify traders that the bears are weakening and the bulls are gaining strength.
If this candlestick forms after an advance, then it is called a hanging man. Even though the bulls won out by the close, this candlestick should serve as an alert to potential weakness.