What It Means
An AMO (After Market Order) is an order you place after regular trading hours end, which the broker queues up and releases to the exchange for the next trading session — useful if you can't be online right when the market opens.
How It Works
Most brokers open an AMO window a few hours after the 3:30 PM close and keep accepting orders into it until early the next morning. For equity, the order gets released into the next day's pre-open session; for F&O, which has no pre-open session, it goes in right at the 9:15 AM open.
Warning
Placing an AMO market order assumes the price won't move much overnight — a large gap up or gap down from news or global cues means your order can execute far from the previous day's closing price.