A long candle represents a single Japanese candlestick where the length (or height) of the candlestick’s body is very long
The large body indicates a huge price move from open to close.
Long white/green candlesticks indicate there is strong buying pressure.
This typically indicates the price is bullish. That said, a long candle should be looked at in the context of the overall price action as opposed to just as a standalone candle.
For example, a long white candle is likely to have more significance if it forms at a major support level.
Long black/red candlesticks indicate there is significant selling pressure. This suggests the price is bearish.