A marubozu is a single candle with no (or very small) shadows — the open equals the low and the close equals the high (bullish marubozu), or the open equals the high and the close equals the low (bearish marubozu). It represents complete dominance by one side of the market.
The marubozu tells a simple but powerful story: one side had total control from open to close. There was no meaningful retracement during the session.
In practice, perfect marubozus (literally zero shadow) are rare. Most traders allow very small shadows — the key criterion is that the body constitutes the vast majority (90%+) of the total range.
A marubozu candle breaking through a key resistance or support level is one of the most convincing breakout confirmations in price action. The complete lack of wicks shows that there was no hesitation at the level.
Marubozus are most useful as qualitative filters — they confirm conviction rather than predict direction.