The object marker N+을/를 is used to indicate that a noun is the object in a sentence. In formal Korean, its use is more strict to clearly define the sentence structure. However, in colloquial speech, this particle is often omitted for brevity or when the context already makes the object clear.
Certain cases allow for contraction, such as
Mark the subject, who or what is doing the action.
Mark the topic, what the sentence is about. The topic of the conversation.
Mark the object, who or what is being acted upon.
Note that these particles can sometimes be dropped in practice and we assume the audience is smart enough to understand the context.