Open is the first traded price of an instrument at the start of a session or chart period. It marks where trading began before price moved on to its high, low, and close.
The period can be a market session, a full trading day, or a chart timeframe such as one minute, one hour, or one week. Open is one of the four points in OHLC data, open, high, low, and close, and it sets the reference you measure the rest of the period against.
Open here means the opening price, not an open position or a pending order, which share the word on trading platforms. Comparing the open with the close tells you who controlled the period: a close above the open shows buyers led, and a close below it shows sellers led.