The subject is what "what practice is' is.
Joko starts off the section, "Practice is very simple. That doesn't mean it won't turn our life around, however."
A simple truth about simple truth.
If I might comment, though, on one part of the Chapter: In one paragraph, Joko seems to be recommending that we actively "label thoughts precisely" during Zazen, as thoughts arise and before releasing them. Then, we should return to 'just sitting." However, if that is what she is recommending, I would have to disagree firmly with that approach and say that such a way is not standard for Shikantaza practice as instructed by most teachers I know. I would not encourage that. Perhaps a little Vipassana influence in her method? I am not sure. When we "just sit," we "just sit" ... we let thoughts go without analysis during Zazen. There is nothing to do or attain in the sitting, nothing to examine or focus on ... and that non-doing and non-focusing is VERY important.
Now, on the other hand, I think her "thought labeling" recommendation is a wonderful thing to do at other times in daily life, as thoughts arise during our busy day ... when tired, hot, a little angry, happy, etc. Just not during Zazen itself. I think, which should have no object or focus to it.