Sorry, let me explain it once more.
Let's say you open a form and your colleague does the same. The quantity is 5.
You want to increase the quantity by five, so you change it to 10, but before you manage to save it, your colleague changes it to 20 (and save the record). Your attempt to save your change will fail, because you're working with outdated data. You wanted to increase quantity by 5, but in fact you would decrease it by 10.
Fortunately there is conflict detection that prevents your from unintentionally deleting changes done by your colleague.