What does this mean?
Maybe I'm supposed to laugh at the thought the editors, whoever they were, couldn't resist the urge to uniformity by putting the "idea for the day" first, in the lesson title, as it is with all the other lessons.
"These thoughts do not mean anything," is the idea for the day, and there it is right at the beginning of the lesson page in the book I'm looking at.
It's possible to look at it as if it were a joke or a lie, but why bother? Let's take it seriously.
This exercise does not begin with, "these thoughts do not mean anything." It begins with "noting the thoughts that are crossing your mind."
The implication is that you are not your thoughts. The suggestion is that thoughts just happen. They appear, move across your mind, and disappear. Like clouds in sky.
For some, this may be a novel idea. Some have realized this intuitively, or recognize it from another source.