A trader has submitted an order to the Exchange to purchase or sell a specific quantity of a financial instrument.
This order has not been filled yet and remains on the Exchanges order book until another party agrees to buy or sell at the offered price.
Example: An open order is an offer to buy or sell a quantity of Bitcoin at a specific price. This offer can be at a significantly lower price than the current market price and in this case the order will not be filled (i.e. actioned), but will remain open, until the price falls to that level (i.e. I want to buy 1 Bitcoin at a price of $5,000; this order will be submitted to the exchange by the trader and will remain open until another party wants to sell Bitcoin at the price of $5,000).