You can buy a one-day bus pass to travel around the city.

pass 在这里是名词，意思是"通行证、票证"，指允许在一定时间内无限制乘坐公交车的票卡。 one-day bus pass 特指"一日公交通票"，即24小时内可以无限次乘坐公交车的票证。