Direct object
The direct object ("objeto directo" in Spanish) is who or what receives the action from the verb in a sentence. For example:
Yo como verduras.
I eat vegetables.
Ayer vi a Pablo.
I saw Pablo yesterday.
We normally ask "what?" or "who?" with regards to the verb to know what the direct object is. If we ask "what do I eat?" (vegetables) and "who did I see?" (Pablo), we know what the direct objects are!
It is important not to confuse el objeto directo with El objeto indirecto which is also affected by the verb in a sentence but in a different way.