An active order is an order that is valid during the trading day or the trading day and additional trading hours.
Where did you hear about active orders?
Every trade you watch or take part in is made up of two separate active orders: one buy order and one sell order. The trade is only completed when the buy order data and the sell order data match. This means that the parameters of each order must be met (in other words, the sell price of one order must match the buy price of another).
What you should know about active orders
There is a huge variety of orders of various types. From simple (for example, one-day orders, the parameters of which are given to the broker, and a sale or purchase can only be made if they are met within one trading day) to more complex ones (including stop-limit orders that combine the features of limit and stop-loss orders).
Any order to buy or sell, regardless of type and parameters, is active if valid during the trading day (or additional trading hours).
An alternative to an active order is an “immediate” order, which the broker must fill almost immediately or it will expire. Such an order does not have enough time to be considered «active».