Success normally comes slowly, sometimes over many years of hard work. An 'overnight success' is when success comes very quickly. One day the person was no one, the next day they are a success. So it is like they found success 'overnight'.

A sudden success. One day you are not particularly successful, the next day you are. You have become successful overnight. Although it is usually an exaggeration, of course. Maybe over a period of several months or perhaps even years, but certainly quick compared to how long most businesses take to build up.

يعني النجاح بيوم وليلة !