A brief explanation of the direct object was given in the section about the reduced direct object?, which I will simply repeat here:
The direct object is what we get if we ask "What does the subject [verbs]?", where [verbs] refers to all verbs in the clause. E.g. "He has made an apple pie". The answer to "What has he made?" is the direct object, in this case 'an apple pie'.
In Dutch, the direct object has not yet agreed on a fixed position in the main clause. Because it is highly inconvenient to have components moving about as they like, we only allow the direct object a position at the beginning or at the very end of the middle part.
We can, however, not randomly place the direct object at the beginning or end of the middle part. The position of the direct object depends on whether it is a specific or a non-specific one.