There are 2 types of orders on the market. If you place a buy order you are offering to purchase an item at a given price. A sell order is an offer to sell at a price you specify.
If you sell immediate - the market will sell your item to the highest buy offer. If you specify a price, it will create a sell order.
Eve is a commodity market and they are cyclical. There is a price above which people will not buy and a price below which they will not sell - the market oscillates between these points. Those of us who manufacture and sell products as our in-game career study these cycles and position our sell orders at a point in the cycle where we are comfortable with the profit margin. Some will chase the market if it goes down from there, some will simply wait for the cycle to turn and let the market come to them.
In addition to producers, there are also traders who use buy orders to acquire product and then resell it at a higher price - profiting from the buy/sell spread. Some will buy in one region and sell in another profiting from inter-regional arbitrage.
It’s complex and can be a lot of fun for those who play Eve primarily for the economic simulation.