出发 means 'to set off, to depart' and specifically emphasizes starting a journey or moving toward a destination. Unlike 离开 (lí kāi), which simply means 'to leave a place' for any reason, 出发 is used when you begin a trip, mission, or route. 启程 (qǐ chéng) and 起程 (qǐ chéng) are more formal, written alternatives.