Buy orders (bids)
price....volume
100......2000
90........3000
80.........100
70........6000
if you have 10,000 coins to sell you either have to place an order at price=100 and volume=10,000 or you have to sell it to these open orders meaning
sell vol=2000 @price=100 (8000 coins left)
sell vol=3000 @price=90 (5000 coins left)
sell vol=100 @price=80 (4900 coins left)
sell vol=4900 @price=70 (0 coins left)
and make (2000*100 + 3000*90 + 100*80 + 4900*70) satoshi for example.
in the platform or even through API you don't need to make each single order, just sell 10000 @70 and it automatically fills these 4 orders.
I think this is already a clear explanation. All the higher price will be served first until it reach the lower price you set. (if it can each it). If there are still coins left to sell, it will automatically posted to the SELL Order at your set price.