A black (red in the figure) candlestick usually means that the closing price of the time period is lower than the opening price of the time period. This candlestick is what you want to see when you have a sell order as you can buy back the currency pair at a much lower price.
The high is the highest price reached during that specific time period and the low is the lowest price reached. Anything that is not in the real body (fat rectangles shown by the green and red body) is known as shadows and depict the movements of the market.
Candlesticks have short and long body. The longer the body is, the higher the buying or selling pressure. Compare the 2 candlesticks at the figure below: