to start a journey:
What time will we have to set off for Grandma’s house tomorrow?
Jenny set off down the road on her new bike.
They’ve just set off on a round-the-world cruise.
to begin a job, task, etc. with a particular aim or goal （怀着目标）开始工作，展开任务，着手
She set out to break the world record.
她一心努力要打破世界纪录。
They succeeded in what they set out to do.
他们实现了既定的目标。