object marker particle
Marks the direct object of a transitive verb.
The particle を marks the thing that receives the action of the verb. In Japanese, the object comes before the verb, so you say 'I apple eat' rather than 'I eat an apple'. It is used with verbs that take an object, like たべる (to eat), のむ (to drink), みる (to see), きく (to listen).
You can think of を as the 'target' of the action. It is not used with intransitive verbs like いく (to go) or ある (to exist), which do not have an object. Also, note that を is sometimes used after places with motion verbs (like こうえんをさんぽする), but for N5, remember it mainly for objects.
When speaking, を is often pronounced 'o', but it is written as を.