A hammer is a single candlestick with a small real body near the top of its range and a long lower wick at least roughly twice the body, appearing after a downtrend. The shape shows price was pushed sharply lower during the period and then bought back up to close near the open — a visible rejection of lower prices.
Traders read the hammer as a potential bottom: the long lower wick is sellers failing to hold the lows. Its bearish mirror, the shooting star, has the same shape inverted at the top of an uptrend. The closely related hanging man is an identically shaped candle that appears after an uptrend and carries the opposite, bearish, implication — proof that context, not shape alone, sets the meaning.
A hammer is a one-candle pattern, so it is weak evidence in isolation and conventionally needs confirmation — a higher close on the next candle, or alignment with a support level. The “2× body” rule and the location filter must be fixed numerically before the pattern can be backtested honestly.