Literally, this is a request to buy or sell, which is valid for one day. You open a transaction to buy or sell and give an indication to the broker that it's a day order. It means that, when you close the trading session using your tool, the broker automatically closes the deal. If you are in the minus or plus, it doesn't matter, the deal is just closed. Why is it needed? The fact is that, in the transfer of a transaction to the next day the broker takes a commission, whereas this type of order guarantees you that no type of trasfer will occur. But please note that MetaTrader 4 terminals there isn't this option, you choose the desired time to close your deal.
Trading in financial markets involves substantial risks, including complete possible loss of investment capital. This activity is not suitable for all investors. High leverage increases the risk (Risk Disclosure).