A day order is an order that stays active only until the end of the trading session in which it was placed. If it has not been executed by the close, the exchange or broker cancels it automatically, and any unfilled part of a partly executed order lapses in the same way. Day is the default duration on most trading platforms, so an order entered with no other instruction is treated as a day order.
The duration of an order is called its time in force. A day order sits between two alternatives. A good-till-cancelled order carries over from one session to the next until it is filled or withdrawn, and an immediate-or-cancel order lasts only an instant. The day setting matters mainly for a limit order or stop order, which may wait for a price that is not reached. It protects a trader from an old order being filled days later, after conditions have changed.