On page 27 Johnston gives a darn good explanation of the wonder of this word:
Not being able to do something yet shows a belief that you will be able to do it at some point in the future. Not being able to do something period does not show that same belief in the ability to learn and grow.
We want to inoculate the children against infection by fixed theories; we want them to say “I’m not good at this yet” and to take steps to change that. Indeed, yet is a key word that we should regularly encourage children to add to their narratives.
If you are interested in reading the thoughts of many wonderful teachers who read this book and wrote about it this summer, Jill Fisch's post will give you a place to begin.