In The Witch of Blackbird Pond, what is John's warning?
You haven't really specified what particular warning you are referring to here, so I have had to assume it is in Chapter 2, when Kit is talking to John Holbrook about her aunt. Kit tries to recall what she has been told about her Aunt Rachel, who she has never actually met, by her mother, and paints a romantic picture of this unknown figure, saying:
"My mother remembered that she was always laughing."
Yet, at this stage, it is John Holbrook who injects a more realistic sober note into Kit's reflections and situation by saying:
"Don't forget, your aunt has been away from England for a long time."
Kit picks up an "intangible warning" in these words, even though she is not able to interpret it. To my mind, John is referring to the realities of Puritan life and the hard work of settlers who had to toil on the land to make ends meet. Such a brutal, harsh life would be more than enough to stop people from laughing all the time.