What does it mean to let nature take its course?

It means that in people's life, there is no need to force many things. As long as we follow the development of things, it will naturally give us a result. For example, if you are lovelorn and you especially want to save your ex, then your ex especially doesn't want to be with you at this time, so you can only do this, and you can only let nature take its course. This is your only choice now, and sometimes it may be in life.