The wick of a candlestick is the thin part that is drawn above and below the body of the candlestick. The highest part of the top candle represents the highest price in the candle. The lowest part of the bottom wick represents the lowest price in the candle.
Wicks are traditionally drawn thinner than bodies because they are less important in traditional candlestick analysis. Traditionally, most of the information about the trading for a day was described by the opening price and the closing price. This tradition started with daily candlesticks, and might not apply to intra-day candles.
On a simple charts, some people draw trendlines that touch the highest point of the top wick or the lowest point of the bottom wick. Other people draw lines that ignore the wicks and touch the edges of the body. On busier charts, lines will often have to go through many wicks in many different places. Some authors call this region “congestion”.