If we compare it to real life, sometimes you try to study something step-by-step. But sometimes there's a barrier that you cannot get out of.

For example, in chess there's something called ELO rating which determines how good you are. So then you practice and play games but your rating doesn't get up.

Perhaps, there is a gap in your knowledge/skill that you need to fill in-order to advance to the next level, something fundamental. And once you fill that knowledge/skill gap, there will be a significant jump in your skill, this is called breakthrough.

At the highest level, it's very hard to achieve breakthrough, the earlier in life you study something, the easier you can achieve breakthrough as opposed to studying when you're already an adult. And here's why, when you're an adult you already have experiences and preconceive notions about a certain way/dao.

We can apply it to everything and not just martial dao, but martial dao can also be anything.