The four prices that summarise any chart period: the first trade, the highest trade, the lowest trade and the last trade.
The parts of a candlestick. One candle sums up a slice of time: the thick real body runs from the opening price to the closing price, and the thin wicks reach out to the highest and lowest prices traded. Colour tells you which way the body ran.
Every bar or candle on a normal chart compresses an entire period into four numbers. The open is the first traded price of the period, the high and low are the extremes, and the close is the last trade before the period ended.
Almost every indicator is built from these. A simple-moving-average usually averages closes, true-range uses the high, low and prior close, and heikin-ashi recomputes all four.
Example: a 5-minute bar with open 100.20, high 100.75, low 100.10, close 100.65 tells you price opened, ran up 55 cents, dipped, and finished near the top. What it cannot tell you is the order those things happened in. That ambiguity is why traders drop to a lower-timeframe when the sequence matters.