The Piercing pattern is a two day trend reversal pattern.
A Piercing pattern happens after a downtrend. Day one has a longer bearish candlestick. Second day candlestick opens below the previous day low and has a closing day price within ( more than 50% into ) day1 real body.
Psychology of Piercing pattern is that on day one bearish candlestick confirms the previous downtrend. On Day 2 price of the security gaps downward to indicate bears are dominating but instead of price continuing to go downward, the price begins to rise and ends up going up more than half of the Day 1 candlestick to indicate that bulls are ready to take the prices higher.
As like any other, piercing patterns require confirmation candlesticks in the following days. If future day candlesticks go below the Day 2 candlestick low then the piercing pattern will be voided.