The particle を is used in Japanese to identify the direct object of a verb. This indicates that the subject (the performer of the action) acts upon the object, which directly receives or is affected by that action. Generally, the noun marked by を is passive in the sentence; it is the thing that the action is happening to.

• 本(ほん)を読(よ)む。
  To read a book. (Object marker)
• ジュースを飲(の)みます。
  To drink juice. (Object marker)

In the examples above, を points to the item that the verb acts upon, which is the standard definition of a direct object. However, を has another common use: to mark a location that is passed through. This occurs with verbs of motion, where the journey or movement through the place is the focus of the action.

• 橋(はし)を渡(わた)る。
  To cross the bridge. (The bridge is the object that 'crossing' is being performed through) (Object marker)
• 森(もり)の中(なか)を散歩(さんぽ)する。
  To take a walk through the forest. (The inside of the forest is the object that 'walking' is being performed through) (Object marker)

With these motion verbs, を indicates the area or path that the action covers from one point to another. This specific usage treats the location itself as the direct object of the movement, as the action (like crossing or walking) is performed directly upon that space.