The particle "を" (pronounced "o") is a Japanese grammatical marker used to indicate the direct object of a verb. It shows what action is being performed on which object. For example, in the sentence "りんごを食べる" (I eat an apple), "りんご" (apple) is the direct object receiving the action of eating. In Japanese, "を" is placed after the noun that serves as the object. It is essential for clarifying the relationship between the verb and the object in a sentence. Understanding how to use "を" correctly helps in constructing clear and meaningful sentences in Japanese.