Like other Korean particles, 를 (reul) is a postposition. It marks the preceding noun as the direct object of the following verb. 를 (reul) is optional if it can be inferred that the noun is the direct object (as opposed to the subject).
를 (reul) is used only after a word ending in a vowel. If the preceding word ends in a consonant, 을 (eul) is used instead: