The orders are partially filled (either on the buy or sell side). What this means is that the shares eligible to transact do so immediately, and the remaining

The orders are partially filled (either on the buy or sell side). What this means is that the shares eligible to transact do so immediately, and the remaining shares sit on the order book and wait for someone to be willing to trade at that price. There is a specific option that you can set (usually called "all or none") that will prohibit partially filling an order and only allow it to execute in entirety.