What it means
Line charts show only closes; bar and candle charts show all four.
In practice
The four prices carry different information. The open and close show where the period started and ended, the high and low show the extremes that were tested. Most indicators are calculated from the close alone.
Daily and weekly opens, highs, lows and closes are also levels in their own right. The previous day's high and low are watched by many intraday traders as reference points and targets.
What to watch for
A line chart hides the highs and lows, so wicks, sweeps and rejections are invisible on it. Use it to see the overall direction, and candles or bars for anything that depends on what happened inside the period.