Learning is a process of adding layers of skill to build up your expertise. In learning Tai Chi we progress from a state of not know anything to knowing something (從無到有). This is normally termed the beginner level.
As you keep drilling you become more familiar with your form. As you move through the learning you begin to accumulate knowledge and experiences. This intermediate level is where you know a lot more (從有到繁).
At the advanced level it is common to learn the high level skills of an art, stuff like secret skills, secret applications, secret forms, secret breath work, and so on. However, in our Tai Chi the advanced level is nothing but the basic level stuff refined over the years.
Instead of accumulating more and more knowledge what we do is to shave off the excess, the deadwood movements that are out of synch or do not comply with the principles. We consolidate the key principles, refine them until instead of many principles they subsume to one overall vital principle (從繁到空).