Object marker
The object particle 을/를 is added to a noun to express that the noun is the object of the sentence. For nouns ending in a vowel, 를 is added, and for nouns ending in a consonant, 을 is added. Common verbs that require this object particle include 먹다 (to eat), 마시다 (to drink), 좋아하다 (to like), 읽다 (to read), 보다 (to see), 만나다 (to meet), 사다 (to buy), 가르치다 (to teach), 배우다 (to learn), and 쓰다 (to write). In colloquial speech, 을/를 is sometimes omitted.
Some notes:
N +하다 -> N하다
When 을/를 is omitted from verbs like 공부를 하다, 수영을 하다, 운동을 하다 and 산책을 하다，the form becomes shortened to a single one-word form: 공부하다, 수영하다, 운동하다 and 산책하다. However, for the verbs 좋아하다 and 싫어하다, because 좋아- and 싫어- are not nouns, the forms 좋아하다 and 싫어하다 are the one-word verb forms.
뭐해요?
The question noun 무엇 can be shortened to 무어 which can be further shortened to 뭐. Thus, the question 뭐를 해요? can be shortened to 뭘 해요? which can be further shortened to 뭐 해요? This form is often used in conversation.
The object marker is used to indicate that a noun is the object in a sentence.
The object marker 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 저 and 를 merging to form 절. source
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