I first heard of this Chinese idiom in the movie The Karate Kid. This effectively translates to:
A simplified explanation of this would be that too much of something can turn out to be bad.
Like many other Chinese proverbs and idioms, this one projects a clearer picture of life to us. Too much of something is really bad.
Imagine yourself working straight for 2 days, maybe for a hackathon or a presentation. Can you cope up with the workload for long?
No. You'll break down. Your body won't be able to support you for long.
Similarly, what happens when you do intensive physical training without taking days off to recover?
You go backwards. Instead of growing muscle and getting stronger, you lose muscle.
You're not a 24x7 ATM machine. You can't go on forever without taking breaks.
While deep work is productive, make sure to cut yourself some slack in between. This will help you fill up your energy tanks and get back to work with more energy.
Also, it's not only about work.
Too much of cake will make you sick. Too much of movies will bore you. And so on.
Do everything. Work, play, love, and eat. Just try to stir things up when you start feeling stagnated.
It helps. Try it.