In forex trading, “fill” refers to the execution of a trade order. When a trader places an order to buy or sell a currency pair, the broker or market maker will try to fill the order at the best available price. If the order is filled, it means that the trade has been executed at the requested price or a better price. If the order is not filled, it may be cancelled or re-submitted with a new price. The speed and reliability of order filling is an important consideration for traders when choosing a forex broker.