A candlestick chart plots price over time using a series of "candles," each representing one time period (a day, an hour, a minute — whatever the chart's interval is set to). Each candle shows four prices at once: the open and close (the body, usually colour-coded — green or hollow for a period that closed higher than it opened, red or filled for the reverse) and the high and low (the thin "wicks" above and below the body).
That's more information per glance than a simple line chart, which only plots the closing price and discards everything else. A long wick shows a price that moved a long way during the period and then reversed before the close, which a line chart erases entirely.
Candlestick patterns — specific shapes and sequences of candles — are the basis of a large body of technical analysis, from single-candle patterns like a "doji" (open and close nearly equal) to multi-candle formations some traders use to read likely short-term direction. Reading the raw price action a candle represents doesn't require knowing any of that vocabulary; the patterns are an additional, optional layer on top.