Inevitable outcome. Following a course of action beyond the ability to change course. The phrase suggests one’s path can be ended and you can turn back but there is a point at which it is too, too late to return. Apollo 13, for example, had a problem on the way to the moon, but they couldn’t simply turn around and come back to Earth because they were past the point of no return, hence the drama.
“At first you can smoke and not get addicted, but after a few months, you reach the point of no return.”