Its a bar where the open and close are both roughly within the same half of the bar, leaving a long 'tail' as some people would call it. The term was developed by Martin Pring. He termed it a Pinocchio bar as although the bar may be a healthy length, suggesting strength, the bar is in fact telling you porkies due to the close in relation to the open. People who use candlesticks would call them hammers, hanging man, blah blah blah.
Am I a walking encyclopedia of trading trivia or what!
I wouldnt use it as a signal in itself, but useful in assessing the general strength of a trend. Useless if it comes in a sideways market imo.