The particle を marks a place through which an action takes place. The verbs are motion verbs such as 歩く(walk), 走る(run), とぶ(fly), 通る(pass through), 散歩する(take a walk), 渡る(cross), 上る(go up), 下りる(go down; get off), 曲がる(make a turn), 回る(go around), and 流れる(flow). These are intransitive verbs (Vi), but they are preceded by を.
歩道を 歩いて ください。
Please walk on the sidewalk.
鳥は 空を とぶ ことが できます。
Birds can fly in the sky.
この 川は せまい 谷を 流れて います。
This river flows through a narrow valley.
Lesson 11-2「わたしは 九時ごろ 家を 出ます。」
I leave home at around 9 o'clock.