Its like if you have purchased 1 BTC from them at say $X and you want to trade it for a high amount say $Y, then you can create an order and set the price limit. Once that order is reached your BTC will be sold and you get $Y. Till that future order is fulfilled you will not be able to use that BTC reserved for that order. You can however cancel the order at any time and use the BTC. This in my opinion is a good initiative and as a user I don't have to keep a tab on the fluctuating prices.