を（移動・起点）
を marking the place moved through / the point departed frommachine translation
FormationNoun indicating a place + を + verb of motion (渡る・歩く・出る…)
You learned を marking the direct object at N5 (パンを食べる). At N4, を has two more uses, both with verbs of MOTION.
First, を marks the place the movement passes through: 橋を渡る (cross a bridge), 公園を散歩する (stroll through the park), 道を歩く (walk along the road), 空を飛ぶ (fly through the sky).
Second, を marks the starting point being left, with verbs like 出る and 降りる: 家を出る (leave the house), 電車を降りる (get off the train). Note that 大学を出る can also mean "to graduate from university."
Most of these verbs are INTRANSITIVE, yet they still take を — an exception worth remembering when you study transitive–intransitive verb pairs.
Examples
橋を渡って、右に曲がってください。
Cross the bridge and then turn right.
毎朝、公園を散歩します。
Every morning I take a walk in the park.
毎日七時に家を出ます。
I leave the house at 7 o'clock every day.