Hi,
As you probably know, this is a famous quotation from the author, G.K. Chesterton.
"The only simplicity that matters is the simplicity of the heart. If that be gone, it can be brought back by no turnips or cellular clothing; but only by tears and terror and the fires that are not quenched".
This is very subjective writing. To me, it means this. Generally, the heart is seen as representing a more basic simpler and truer part of our character than the brain. Chesterton spoke of this when he wrote
Simplicity of character is no hindrance to the subtlety of intellect'
In your quote, Chesterton is saying that simpleness in a person's heart rather than in his brain, simple emotions and values like love, honesty and trust, are the most important elements in his character.
Perhaps you agree? Or perhaps not? It's his opinion.
Best wishes, Clive